Remove warnings at the root

[SamFerracin]

Description of the Change:
Removes ComplexWarning at the root (inside the backend) rather than in the functions that require casting

[codecov]

Codecov Report

Merging #307 (7e9c968) into develop (fba4c72) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #307   +/-   ##
========================================
  Coverage    82.33%   82.33%           
========================================
  Files           60       60           
  Lines         4466     4466           
========================================
  Hits          3677     3677           
  Misses         789      789           

Files	Coverage Δ
mrmustard/math/backend_numpy.py	100.00% <ø> (ø)
mrmustard/math/backend_tensorflow.py	100.00% <ø> (ø)

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[sduquemesa]

Thanks @SamFerracin!

I wonder if this PR is also a good opportunity to disable tensorflow's CUDA warning messages — those that appear when importing TF and no GPU is available. What do you think? One option is to just disable them and the other is to install tensorflow-cpu instead of the full tensorflow package that includes GPU support. Also, tensorflow-cpu's wheel is smaller than the full tensorflow meaning faster setup times.

[SamFerracin]

@sduquemesa That's a great idea. i hate those warnings. Let me look into it :)

[ziofil]

approved, up to you to cover casting to int32 correctly