make \ type stable

[araujoms]

Closes #111

Since UMFPACK is not going to add support for Float32 anytime soon I think the only solution is to manually convert back. Also, since the code for dealing with the output of lu is not generic, there is no point in converting back the decomposition itself, so I'm just converting the result of lu(A) \ b to the appropriate type.

I also did the analogous change to achieve type stability with Float16. There's also nothing that can be done about CHOLMOD, but for SPQR the situation is better: the code around it is generic, so I just converted back qr(A) itself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.31%. Comparing base (2b32b60) to head (b5450f8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #684      +/-   ##
==========================================
+ Coverage   84.28%   84.31%   +0.03%     
==========================================
  Files          13       13              
  Lines        9340     9347       +7     
==========================================
+ Hits         7872     7881       +9     
+ Misses       1468     1466       -2     

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