Support keywords on material cards

[paulromano]

The material card in MCNP allows a number of keywords to be specified. They can be given anywhere on the card so long as they don't separate a nuclide/density pair (i.e., "92235 NLIB=80c 0.02" is not allowed). This PR allows these keywords to be parsed correctly, although they are otherwise ignored.

Closes #40

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.23%. Comparing base (5926b50) to head (458a63a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
+ Coverage   87.06%   87.23%   +0.17%     
==========================================
  Files           3        3              
  Lines         742      752      +10     
  Branches      197      198       +1     
==========================================
+ Hits          646      656      +10     
  Misses         52       52              
  Partials       44       44              

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