Add tests for Proposal

[mj-will]

Add complete tests for nessai.proposal.base.Proposal.

Also changes Proposal to inherit from abc.ABC so that draw is an abstract method.

[codecov-commenter]

Codecov Report

Merging #92 (c2b5b65) into main (e7f3b97) will increase coverage by 0.34%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
+ Coverage   91.68%   92.03%   +0.34%     
==========================================
  Files          29       29              
  Lines        3487     3489       +2     
==========================================
+ Hits         3197     3211      +14     
+ Misses        290      278      -12     

Flag	Coverage Δ
unittests	92.03% <100.00%> (+0.34%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nessai/proposal/base.py	97.43% <100.00%> (+29.01%)	⬆️
nessai/proposal/analytic.py	90.32% <0.00%> (-9.68%)	⬇️
nessai/proposal/rejection.py	97.91% <0.00%> (-2.09%)	⬇️
nessai/proposal/flowproposal.py	96.84% <0.00%> (-0.87%)	⬇️

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