Migrate legacy HDF5 MCMC sample IDs

[BJMCox]

Fixes #553.

Fill missing legacy walker and proposal IDs with Int32(1), matching single-walker files and the ordinary proposal index.

Main code: +3/-3.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.59%. Comparing base (6e8795f) to head (a96c127).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #561      +/-   ##
==========================================
+ Coverage   65.45%   65.59%   +0.14%     
==========================================
  Files         121      121              
  Lines        7060     7060              
==========================================
+ Hits         4621     4631      +10     
+ Misses       2439     2429      -10     

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