Add isoelectronic sequence property and helper function

[wtbarnes]

Ions in the same isoelectronic sequence (i.e. those that have the same number of bound electrons) often share common properties despite having different charge states and being from different ions. As such, it is often useful to know to which isoelectronic sequence an ion belongs or to group ions of a single sequence together.

As such, this PR adds two features:

• Add a isoelectronic_sequence property to fiasco.Ion
• Add a helper function fiasco.get_isoelectronic_sequence that returns all ions in CHIANTI that fall within a given isoelectronic sequence.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.92%. Comparing base (545da0a) to head (d9bdfaf).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #320      +/-   ##
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+ Coverage   90.47%   90.92%   +0.45%     
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  Files          40       40              
  Lines        3369     3384      +15     
==========================================
+ Hits         3048     3077      +29     
+ Misses        321      307      -14     

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