Cupy matrices

[BrunoLiegiBastonLiegi]

This attempts to reintroduce the cupy matrices as default for the cupy backend, as discussed in #196. It also fixes a bunch of other tests that were failing with cupy.

edit: The tests seem to run fine on my local GPU, however I had to implement a maybe redundant class CustomCupyMatrices, which may be unnecessary under a more suitable inheritance chain of the matrices defined inside backends,matrices.py

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (b782805) to head (ecf7312).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #199   +/-   ##
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  Coverage   99.91%   99.91%           
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  Files          12       12           
  Lines        1115     1115           
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  Hits         1114     1114           
  Misses          1        1           

Flag	Coverage Δ
unittests	99.91% <100.00%> (ø)

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