Marcs model ingestion bugfix

[jvshields]

This is a small bugfix change. I had implemented a less-than-ideal way of ingesting marcs models and calculating atomic mass fractions that set atomic numbers greater than the final atomic number to nan. These get truncated again later on, so there's no real reason to do this and it just makes pandas spit out a bunch of warnings for (understandably) bad code.

There's also two sourcery facelifts that just make the code a little easier to read.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b560322) 66.73% compared to head (62a209f) 66.66%.

Files	Patch %	Lines
stardis/io/model/marcs.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #147      +/-   ##
==========================================
- Coverage   66.73%   66.66%   -0.07%     
==========================================
  Files          27       27              
  Lines        1016     1011       -5     
==========================================
- Hits          678      674       -4     
+ Misses        338      337       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.