Fix DecFileParser.expand_decay_modes method

[eduardo-rodrigues]

Hello @admorris, a while back you had reported that for example using the LHCb decay file Bc_excitedtrueCharmoniumTauNu,charmonium+=cocktail,mm,pipipi=DecProdCut,CntCut.dec the new DecFileParser.expand_decay_modes(...) function was not correctly resolving all aliases. You say "so far I've only seen it with neutral unflavoured mesons, so it might be a clash beween the unaliasing and charge-conjugation".

It turns out the issue is that aliases were only resolved for decay mothers, not the final-state particles. I fixed this now via this PR, and added a comprehensive set of tests + improved the docs a bit (the fix itself is a one-liner).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.04%. Comparing base (e6e1fe1) to head (02034f9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #634   +/-   ##
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  Coverage   98.04%   98.04%           
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  Files           6        6           
  Lines        1276     1276           
=======================================
  Hits         1251     1251           
  Misses         25       25           

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