Refining Continuous Operators: make them Jax-compatible pytrees

[PhilipVinc]

As the Jax operator interface for discrete operator is turning out very successful and extremely useful and easy to use, I'm thinking about turning ContinuousOperators into something very similar.

This also makes it much simpler to use them in custom codes.

This should be very low effort, as they already are simple jax transformations and are much simpler than their discrete counterparts...

Should be further discussed.

cc @gpescia

[codecov]

Codecov Report

Attention: Patch coverage is 60.86957% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 51.34%. Comparing base (28866b9) to head (06401cb).

Files	Patch %	Lines
netket/operator/_potential.py	47.82%	12 Missing ⚠️
netket/operator/_sumoperators.py	60.00%	8 Missing ⚠️
netket/operator/_kinetic.py	75.00%	5 Missing ⚠️
netket/vqs/mc/mc_state/expect.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1744       +/-   ##
===========================================
- Coverage   82.75%   51.34%   -31.41%     
===========================================
  Files         300      300               
  Lines       18300    18332       +32     
  Branches     3483     3485        +2     
===========================================
- Hits        15144     9413     -5731     
- Misses       2477     8226     +5749     
- Partials      679      693       +14     

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