[TST] Add test docstrings to ml, biology, chemistry modules

[hectormz]

PR Description

Please describe the changes proposed in the pull request:

• Adds docstrings to tests in following modules:
  • ml
  • biology
  • chemistry

I did not write any of the original functions or tests, and am open to feedback for better descriptions.

This PR partially addresses #306.

PR Checklist

Please ensure that you have done the following:

1. PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

2. If you're not on the contributors list, add yourself to AUTHORS.rst.

3. Add a line to CHANGELOG.rst under the latest version header (i.e. the one that is "on deck") describing the contribution.
   • Do use some discretion here; if there are multiple PRs that are related, keep them in a single line.

Quick Check

To do a very quick check that everything is correct, follow these steps below:

• Run the command make check from pyjanitor's top-level directory. This will automatically run:
  • black formatting
  • flake8 checking
  • running the test suite
  • docs build

Once done, please check off the check-box above.

If make check does not work for you, you can execute the commands listed in the Makefile individually.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[codecov]

Codecov Report

Merging #649 into dev will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #649   +/-   ##
=======================================
  Coverage   92.36%   92.36%           
=======================================
  Files          16       16           
  Lines         524      524           
=======================================
  Hits          484      484           
  Misses         40       40           

[ericmjl]

Only one minor change requested, @hectormz. As always, you've done a wonderful job here 😄.

[hectormz]

@ericmjl thanks for the feedback, just updated it!

[ericmjl]

Thanks @hectormz!

[hectormz]

More to follow...eventually...

[ericmjl]

No pressure! 😄

[hectormz]

Bitesize. It's actually also a really nice to gain some more familiarity with parts of the codebase I'm not familiar with. So good for new and old contributors.