Add constant volume mode

[mj-will]

Add a constant volume mode to FlowProposal. In this mode the radius of the latent contour is fixed to the q'th quantile of an n-dimensional Gaussian, which by default is 0.95.

[codecov-commenter]

Codecov Report

Merging #125 (420d254) into main (b21ac5d) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
+ Coverage   94.78%   94.81%   +0.02%     
==========================================
  Files          39       39              
  Lines        3569     3588      +19     
==========================================
+ Hits         3383     3402      +19     
  Misses        186      186              

Flag	Coverage Δ
unittests	94.81% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
nessai/utils/__init__.py	100.00% <ø> (ø)
nessai/proposal/flowproposal.py	96.91% <100.00%> (+0.07%)	⬆️
nessai/utils/sampling.py	100.00% <100.00%> (ø)

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