Updated fractions

[alexmaragko]

• Replaced ionization fraction with charge fractions.
• Replaced large fraction with size fractions.
• Computed nc and added to fits format as an extension.
• Added the ionized-over-neutral PAH fraction extension.
• Updated plots with the above.
• Updated tests.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.29730% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 91.98%. Comparing base (486f85e) to head (939a542).

Files	Patch %	Lines
pypahdb/decomposer_base.py	94.91%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   90.88%   91.98%   +1.09%     
==========================================
  Files           4        4              
  Lines         450      524      +74     
==========================================
+ Hits          409      482      +73     
- Misses         41       42       +1     

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