Support one-to-many reparameterisations

[mj-will]

This PR adds support for one-to-many reparameterisations, e.g. for the reparameterisation proposed in mj-will/nessai-gw#1.

The main change is to skip the steps that ensure the reparameterisation are invertible if one_to_one is False.

[johnveitch]

Just a clarification on the warning message

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.60%. Comparing base (7836002) to head (fd057d3).
Report is 112 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #418   +/-   ##
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  Coverage   93.59%   93.60%           
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  Files          69       69           
  Lines        6748     6755    +7     
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+ Hits         6316     6323    +7     
  Misses        432      432           

Flag	Coverage Δ
unittests	93.60% <100.00%> (+<0.01%)	⬆️

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