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 (8302937).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #199   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          12       12           
  Lines        1115     1116    +1     
=======================================
+ Hits         1114     1115    +1     
  Misses          1        1           

Flag	Coverage Δ
unittests	99.91% <100.00%> (+<0.01%)	⬆️

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[andrea-pasquale]

Before I forget it would be nice to modify these lines

qibojit/src/qibojit/tests/test_backends.py

Lines 122 to 124 in 8302937

	def test_metabackend_list_available():
	available_backends = dict(zip(("numba", "cupy", "cuquantum"), (True, False, False)))
	assert MetaBackend().list_available() == available_backends

whenever I test locally I need to edit them. Do you think that we can add some try-except blocks to check which gpu backends are available?

[BrunoLiegiBastonLiegi]

yeah, definitely.