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 :warning:

Additional details and impacted files

```diff @@ 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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