ENH: add minipcn mcmc proposal

[mj-will]

Add an MCMC-based flow proposal that uses minipcn.

To-Do:

• Large scale testing
• Unit tests

[codecov]

Codecov Report

❌ Patch coverage is 19.44444% with 58 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.57%. Comparing base (02ff8fb) to head (95f53fd).

Files with missing lines	Patch %	Lines
src/nessai/proposal/flowproposal/mcmc.py	17.14%	58 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #469      +/-   ##
==========================================
- Coverage   94.34%   93.57%   -0.78%     
==========================================
  Files          68       69       +1     
  Lines        6880     6952      +72     
==========================================
+ Hits         6491     6505      +14     
- Misses        389      447      +58     

Flag	Coverage Δ
unittests	93.57% <19.44%> (-0.78%)	⬇️

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