Refactor flowproposal into a submodule

[mj-will]

Whilst working on a version of FlowProposal that uses MCMC I realised it would be a lot easier if it could inherit from a base class that had also the training/reparameterisations but didn't include e.g. the options related to the latent contour. This PR refactors nessai.proposal.flowproposal into a submodule to hopefully make things simpler going forward.

I've tried to keep the commits as simple as possible, so looking at fd9ff51 might be the easiest way to see that things haven't changed significantly (the same tests with minimal changes still pass)

Other changes

• Remove the deprecated rejection_sampling method
• Add configure_poolsize and change configure_population
• Include the constant volume mode configuration in FlowProposal.set_rescaling
• Rename N to n_samples
• Add an inverse method FlowModel

To-Do

• Refactor tests to distinguish between the two classes

[codecov]

Codecov Report

Attention: Patch coverage is 97.81250% with 7 lines in your changes missing coverage. Please review.

Project coverage is 93.69%. Comparing base (3efb413) to head (4b67569).
Report is 111 commits behind head on main.

Files with missing lines	Patch %	Lines
nessai/proposal/flowproposal/flowproposal.py	97.08%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
+ Coverage   93.60%   93.69%   +0.09%     
==========================================
  Files          69       71       +2     
  Lines        6755     6807      +52     
==========================================
+ Hits         6323     6378      +55     
+ Misses        432      429       -3     

Flag	Coverage Δ
unittests	93.69% <97.81%> (+0.09%)	⬆️

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