Handle multi-particle importance syntax

[paulromano]

As described in #55, when a cell importance lists multiple particles, right now it crashes the converter. This is permitted as described in the MCNP manual (emphasis added):

Any particle symbol from Table 4.3. May also be a list of particle symbols separated by commas as long as the importance are the same for the desired importance are the same for the different listed particle types.

This PR adds support for this syntax along with corresponding tests.

Closes #55

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.86%. Comparing base (f0145bf) to head (3f8a043).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #56   +/-   ##
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  Coverage   86.86%   86.86%           
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  Files           3        3           
  Lines         731      731           
  Branches      191      191           
=======================================
  Hits          635      635           
  Misses         52       52           
  Partials       44       44           

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