Advance analysis and visualisation modules

[Christovis]

This PR adds:

crop_by_subject
get_subjects
get_subjectscount

[codecov-commenter]

Codecov Report

Merging #490 (b50c769) into main (285b113) will decrease coverage by 2.13%. The diff coverage is 69.58%.

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
- Coverage   76.96%   74.82%   -2.14%     
==========================================
  Files          20       21       +1     
  Lines        2830     3019     +189     
==========================================
+ Hits         2178     2259      +81     
- Misses        652      760     +108     

Flag	Coverage Δ
unittests	74.82% <69.58%> (-2.14%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
bigbang/analysis/listserv.py	64.35% <53.57%> (-16.12%)	:arrow_down:
bigbang/listserv.py	79.60% <75.86%> (-0.63%)	:arrow_down:
bigbang/io.py	82.40% <80.64%> (-3.32%)	:arrow_down:
bigbang/analysis/utils.py	97.72% <97.72%> (ø)
tests/analysis/test_listserv.py	98.26% <100.00%> (-1.74%)	:arrow_down:
tests/unit/test_listserv.py	100.00% <100.00%> (ø)

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[sbenthall]

What do these functions do?

Very soon I'll be making a PR that adds Sphinx configured with numpydoc formatted autodocs.

Meaning that the docstrings on existing and new functions should be in numpydoc format.

https://numpydoc.readthedocs.io/en/latest/format.html

So from my perspective it would improve this PR a lot if the new methods had numpydoc formatted docstrings.

Another nice thing would be a Jupyter Notebook demonstrating the functionality. I see the PR includes some interesting looking plots but I have no idea how to review them.