multiplication specialization for RectDiagonal and DiagonalFill

[Wu-Chenyang]

This PR implements the multiplication specialization for RectDiagonal and DiagonalFill in accordance to the following table.

| Left \ Right Multiplier | RectDiagonal | RectDiagonalFill | RectDiagonalZeros | RectDiagonalOnes | Diagonal | DiagonalFill | DiagonalZeros | DiagonalOnes | Zeros | Ones | Fill | Mat | Vec | ZerosVec | OnesVec | FillVec | Adjoint/Transpose Vec | Adjoint/Transpose ZerosVec | Adjoint/Transpose OnesVec | Adjoint/Transpose FillVec |
|------------------------|--------------|------------------|-------------------|------------------|-----------|--------------|---------------|--------------|--------|------|------|-----|-----|-----------|----------|----------|---------------------|------------------------|------------------------|------------------------|
| RectDiagonal | RectDiagonal | RectDiagonal | Zeros | RectDiagonal | RectDiagonal | RectDiagonal | Zeros | RectDiagonal | Zeros | Mat | Mat | Mat | Vec | Zeros | Vec | Vec | Mat | Zeros | Mat | Mat |
| RectDiagonalFill | RectDiagonal | RectDiagonal | Zeros | RectDiagonal | RectDiagonal | RectDiagonalFill | Zeros | RectDiagonalFill | Zeros | Mat | Mat | Mat | Vec | Zeros | Vec | Vec | Mat | Zeros | Mat | Mat |
| RectDiagonalZeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros |
| RectDiagonalOnes | RectDiagonal | RectDiagonal | Zeros | RectDiagonal | RectDiagonal | RectDiagonalFill | Zeros | RectDiagonalOnes | Zeros | Mat | Mat | Mat | Vec | Zeros | Vec | Vec | Mat | Zeros | Mat | Mat |
| Diagonal | RectDiagonal | RectDiagonal | Zeros | RectDiagonal | Diagonal | Diagonal | DiagonalZeros | Diagonal | Zeros | Mat | Mat | Mat | Vec | Zeros | Vec | Vec | Mat | Zeros | Mat | Mat |
| DiagonalFill | RectDiagonal | RectDiagonalFill | Zeros | RectDiagonalFill | Diagonal | DiagonalFill | DiagonalZeros | DiagonalFill | Zeros | Fill | Fill | Mat | Vec | Zeros | Fill | Fill | Mat | Zeros | Fill | Fill |
| DiagonalZeros | Zeros | Zeros | Zeros | Zeros | DiagonalZeros | DiagonalZeros | DiagonalZeros | DiagonalZeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros |
| DiagonalOnes | RectDiagonal | RectDiagonalFill | Zeros | RectDiagonalOnes | Diagonal | DiagonalFill | DiagonalZeros | DiagonalOnes | Zeros | Ones | Fill | Mat | Vec | Zeros | Ones | Fill | Mat | Zeros | Ones | Fill |
| Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros |
| Ones | Mat | Mat | Zeros | Mat | Mat | Fill | Zeros | Ones | Zeros | Fill | Fill | Mat | Fill | Zeros | Fill | Fill | Mat | Zeros | Ones | Fill |
| Fill | Mat | Mat | Zeros | Mat | Mat | Fill | Zeros | Fill | Zeros | Fill | Fill | Mat | Fill | Zeros | Fill | Fill | Mat | Zeros | Fill | Fill |
| Mat | Mat | Mat | Zeros | Mat | Mat | Mat | Zeros | Mat | Zeros | Mat | Mat | Mat | Vec | Zeros | Vec | Vec | Mat | Zeros | Mat | Mat |
| Vec | Mat | Mat | Zeros | Mat | Mat | Mat | Zeros | Mat | Zeros | Fill | Fill | Mat | \ | \ | \ | \ | Mat | Zeros | Mat | Mat |
| ZerosVec | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | \ | \ | \ | \ | Zeros | Zeros | Zeros | Zeros |
| OnesVec | Vec | Vec | Zeros | Vec | Vec | Fill | Zeros | Ones | Zeros | Ones | Fill | Mat | \ | \ | \ | \ | Mat | Zeros | Ones | Fill |
| FillVec | Vec | Vec | Zeros | Vec | Vec | Fill | Zeros | Fill | Zeros | Fill | Fill | Mat | \ | \ | \ | \ | Mat | Zeros | Fill | Fill |
| Adjoint/Transpose Vec | Mat | Mat | Zeros | Mat | Mat | Mat | Zeros | Mat | Zeros | Mat | Mat | Mat | Number | Number | Number | Number | \ | \ | \ | \ |
| Adjoint/Transpose ZerosVec | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Zeros | Number | Number | Number | Number | \ | \ | \ | \ |
| Adjoint/Transpose OnesVec | Mat | Mat | Zeros | Mat | Mat | Fill | Zeros | Ones | Zeros | Fill | Fill | Mat | Number | Number | Number | Number | \ | \ | \ | \ |
| Adjoint/Transpose FillVec | Mat | Mat | Zeros | Mat | Mat | Fill | Zeros | Fill | Zeros | Fill | Fill | Mat | Number | Number | Number | Number | \ | \ | \ | \ |

It also implements the scalar multiplication of RectDiagonal mentioned in #107.
Incidentally, it implements some other related operations including:

• Transpose and Adjoint of RectDiagonal
• Out products vec * adjoint(vec) and vec * transpose(vec) for AbstractFillVector
• Multiplication specialization for OneElementMatrix and DiagonalFill

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.38%. Comparing base (877375f) to head (8c374a1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
- Coverage   99.48%   99.38%   -0.11%     
==========================================
  Files           8        8              
  Lines        1157     1293     +136     
==========================================
+ Hits         1151     1285     +134     
- Misses          6        8       +2     

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[Wu-Chenyang]

There are two possible solutions for resolving the failed integration tests:

1. Implement methods for the multiplication of DiagonalFill and LayoutMatrix in an extension file
2. Change the AbstractMatrix at fillalgebra.jl#L644 to Matrix.

Tell me if any of these options is preferred, and I will make the changes.

Besides, the pull request is now ready for review. Please let me know if any of the code is confusing or requires modification.