fix missing C2 in vald

[jvshields]

Fixing a small bug in the vald molecule linelist parsing that caused it to miss diatomic molecules that have the same base atom - mostly just C2.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.07%. Comparing base (3403655) to head (1a2cfe4).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #422      +/-   ##
==========================================
+ Coverage   66.68%   67.07%   +0.38%     
==========================================
  Files          46       46              
  Lines        3497     3547      +50     
==========================================
+ Hits         2332     2379      +47     
- Misses       1165     1168       +3     

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[andrewfullard]

A comment on what the regex means would be nice