Improve realnvp exceptions and tests

[mj-will]

Make the exceptions raised for incorrect input values more verbose and add tests for parts of the code that were not being tested.

[codecov-commenter]

Codecov Report

Merging #109 (be0009e) into main (230ef47) will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   94.41%   94.64%   +0.22%     
==========================================
  Files          39       39              
  Lines        3547     3549       +2     
==========================================
+ Hits         3349     3359      +10     
+ Misses        198      190       -8     

Flag	Coverage Δ
unittests	94.64% <100.00%> (+0.22%)	⬆️

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Impacted Files	Coverage Δ
nessai/flows/realnvp.py	100.00% <100.00%> (+13.33%)	⬆️

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