Plot revamp

[mwilensky768]

Redo a few things in the plotting code to avoid using deprecated matplotlib functionality. Would close #105

Description

matplotlib warns against using FixedFormatter for ticks without using FixedLocator. Even though it's never been a problem so far, I've made the plotting code explicitly use FixedLocator when using FixedFormatter (which is implicitly called when handing e.g. set_xticklabels a list or other sequence). It also warns against modifying the globally registered colormap. I've thus made all colormap alternations operate on a copy of the global cmap object.

Checklist:

Main Code Body Checklist:

• [x] Add or update docstring related to code change
• [ ] Add a unit test that verifies the efficacy of the code
• [x] Write a tutorial if the feature would be useful to others to use
• [x] Update CHANGELOG.md

Scripts Checklist:

• [ ] Add useful help strings for any arguments that are parsed
• [ ] Manually check that the script runs and gives proper results
• [ ] Update CHANGELOG.md

[codecov-commenter]

Codecov Report

Merging #115 (68cc13a) into master (390aa9b) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #115   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          854       862    +8     
=========================================
+ Hits           854       862    +8     

Impacted Files	Coverage Δ
SSINS/Catalog_Plot.py	100.00% <ø> (ø)
SSINS/plot_lib.py	100.00% <100.00%> (ø)

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