add real_gaussian_integral function

[sylviemonet]

Context: Add the code for real_gaussian_integral to help the from_quadrature() function.

Description of the Change: The mathematical formula corresponds to the one on the overleaf Appendix.

TODO list:

• Add a test for this? (No idea how to test this.)
• Move the complex gaussian integral into this file as well?

Benefits: We can deal with the basis change with a real integral.

Possible Drawbacks: We may need a for loop outside to loop on the integral with different values of b. (Need to be discussed!)

Related GitHub Issues:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.01%. Comparing base (fbb6479) to head (c095dfe).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #371      +/-   ##
===========================================
- Coverage    87.25%   87.01%   -0.24%     
===========================================
  Files           79       80       +1     
  Lines         5956     5978      +22     
===========================================
+ Hits          5197     5202       +5     
- Misses         759      776      +17     

Files	Coverage Δ
mrmustard/physics/bargmann.py	100.00% <ø> (ø)
mrmustard/physics/gaussian_integrals.py	100.00% <100.00%> (ø)
mrmustard/physics/representations.py	99.44% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[SamFerracin]

Nice!

• Why don't you leave it in bargmann.py, alongside the complex gaussian integral? I think it makes sense there
• Tests: I would just do as in test_complex_gaussian_integral, where you pass a triple and check that the result is the one you expect (but you may have to hardcode A3, b3, c3, instead of getting them from another triple

[ziofil]

great!

[SamFerracin]

Looks good :)

Just make sure that you can see the gaussian integrals in the docs (I can help with that)