New poly*exp ansatz

[kaspernielsen96]

Context:
Implementing the new poly*exp ansatz.
Description of the Change:
For now just writing and testing the ansatz, but NOT yet changing the Bargmann representation or anything to use the ansatz.
Benefits:
Many once implemented

[codecov]

Codecov Report

Attention: Patch coverage is 95.13889% with 7 lines in your changes missing coverage. Please review.

Project coverage is 88.67%. Comparing base (c483c0e) to head (ac4081a).

Files	Patch %	Lines
mrmustard/physics/ansatze.py	95.07%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #432      +/-   ##
===========================================
+ Coverage    88.54%   88.67%   +0.13%     
===========================================
  Files           98       98              
  Lines         6799     6940     +141     
===========================================
+ Hits          6020     6154     +134     
- Misses         779      786       +7     

Files	Coverage Δ
mrmustard/math/backend_manager.py	98.16% <100.00%> (+<0.01%)	⬆️
mrmustard/math/backend_numpy.py	100.00% <ø> (ø)
mrmustard/math/backend_tensorflow.py	100.00% <ø> (ø)
mrmustard/math/lattice/steps.py	35.00% <ø> (ø)
mrmustard/math/lattice/strategies/vanilla.py	100.00% <ø> (ø)
mrmustard/physics/gaussian_integrals.py	98.44% <ø> (ø)
mrmustard/physics/ansatze.py	96.12% <95.07%> (-0.59%)	⬇️

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[elib20]

Nice work!

[elib20]

reapproved!