Add split and xdist for pytest

[kaitj]

Proposed changes
The current tests take 10+ minutes, with the majority of this due to the generation and permutation of different transformations, as well as generation of complete AfidSets. This PR adds pytest-split and pytest-xdist in an attempt to speed up the testing (which would presumably get longer with more code added). I've also added the .test_duration generation as part of the poe test task, however the onus would still be on the contributor to run this.

Types of changes
What types of changes does your code introduce? Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (if none of the other choices apply)

Checklist
Put an x in the boxes that apply. You can also fill these out after creating the PR. If you are unsure about any of the choices, don't hesitate to ask!

• Changes have been tested to ensure that fix is effective or that a feature works.
• Changes pass the unit tests
• Code has been run through the poe quality task
• I have included necessary documentation or comments (as necessary)
• Any dependent changes have been merged and published

Notes
All PRs will undergo the unit testing before being reviewed. You may be requested to explain or make additional changes before the PR is accepted.

[codecov]

Codecov Report

Merging #43 (8a31c8c) into main (415cf15) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #43   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines          653       654    +1     
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+ Hits           653       654    +1     

Components	Coverage Δ
afids_utils/afids.py	∅ <ø> (∅)
afids_utils/ext	100.00% <ø> (ø)
afids_utils/transforms.py	100.00% <ø> (ø)