Forward blocks to parent in block-blockrange views

[jishnub]

After this,

julia> S = sprand(Float64,4,4,0.2);

julia> B = mortar(fill(S,3,3));

julia> SB = view(B, Block(1), BlockRange(2:3))
4×8 view(::BlockMatrix{Float64, Matrix{SparseMatrixCSC{Float64, Int64}}, Tuple{BlockedUnitRange{Vector{Int64}}, BlockedUnitRange{Vector{Int64}}}}, BlockSlice(Block(1),1:4), BlockSlice(BlockRange(2:3),5:1:12)) with eltype Float64 with indices Base.OneTo(4)×1:1:8:
 0.0       0.0      0.0  0.0  │  0.0       0.0      0.0  0.0
 0.158791  0.0      0.0  0.0  │  0.158791  0.0      0.0  0.0
 0.0       0.0      0.0  0.0  │  0.0       0.0      0.0  0.0
 0.0       0.96011  0.0  0.0  │  0.0       0.96011  0.0  0.0

julia> view(SB, Block(1), Block(2))
4×4 SparseMatrixCSC{Float64, Int64} with 2 stored entries:
  ⋅         ⋅        ⋅    ⋅ 
 0.158791   ⋅        ⋅    ⋅ 
  ⋅         ⋅        ⋅    ⋅ 
  ⋅        0.96011   ⋅    ⋅ 

The view is forwarded to the parent block array, even when the parentindices of the view is a mix of Block and BlockRanges. On master, this works only if the parentindices are all BlockRanges. This, using lmul! instead of LMul in the matrix-multiplication method, makes the following faster:

julia> S = sprand(Float64,400,400,0.01);

julia> B = mortar(fill(S,3,3));

julia> U = UpperTriangular(B);

julia> v = Float64[1:size(B,2);];

julia> @btime $U * $v;
  10.724 ms (42 allocations: 12.42 KiB) # master
  2.999 ms (30 allocations: 11.55 KiB) # PR

[codecov]

Codecov Report

Merging #282 (d316e9c) into master (a930509) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #282      +/-   ##
==========================================
- Coverage   92.18%   92.16%   -0.02%     
==========================================
  Files          16       16              
  Lines        1484     1481       -3     
==========================================
- Hits         1368     1365       -3     
  Misses        116      116              

Impacted Files	Coverage Δ
src/BlockArrays.jl	100.00% <ø> (ø)
src/blockarray.jl	96.29% <100.00%> (ø)
src/blocklinalg.jl	95.28% <100.00%> (+0.83%)	⬆️
src/views.jl	85.88% <100.00%> (-2.22%)	⬇️

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