Refactor of PBC distance functions

[w-k-jones]

Following on from #384 , I've carried out a more substantial refactoring of the PBC distance calculation functions. This includes improved validation of PBC keyword parameters, and optimisation of the calc_distance_coords_pbc to reduce the amount of work performed each time it is called. build_distance_function should now be the uniform way to create a PBC dist func across all routines

Change in performance of tracking with PBCs (for a 24x1200x3600 dataset):

No PBCs: 4.41 s ± 445 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
Pre-refactor: 52 s ± 4.26 s per loop (mean ± std. dev. of 7 runs, 1 loop each)
Post-refactor: 23.7 s ± 570 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

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[github-actions]

Linting results by Pylint:

Your code has been rated at 8.72/10 (previous run: 8.72/10, +0.00)
_{The linting score is an indicator that reflects how well your code version follows Pylint’s coding standards and quality metrics with respect to the RC_v1.5.x branch.
A decrease usually indicates your new code does not fully meet style guidelines or has potential errors.}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.21%. Comparing base (08f84b2) to head (319d7a3).
Report is 214 commits behind head on RC_v1.5.x.

Additional details and impacted files

@@              Coverage Diff              @@
##           RC_v1.5.x     #386      +/-   ##
=============================================
+ Coverage      56.91%   57.21%   +0.29%     
=============================================
  Files             20       20              
  Lines           3440     3459      +19     
=============================================
+ Hits            1958     1979      +21     
+ Misses          1482     1480       -2     

Flag	Coverage Δ
unittests	57.21% <100.00%> (+0.29%)	⬆️

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[freemansw1]

Thanks for doing this refactor, @w-k-jones. Looks good to me.

[harrietgilmour]

This all looks good - thanks @w-k-jones! Should be ready to merge now