Fix minimum distance filtering for varying vertical coordinates

[w-k-jones]

Resolves #430 and #413

The order of coordinate interpolation and minimum distance filtering has been swapped, so that the z coordinate can be used for finding the spacing of features rather than assuming a fixed vertical grid spacing. The filter_min_distance has also been fixed so that the provided z coordinate will be used, rather than always using dz. Incidentally, another bug was fixed in feature_detection_multithreshold_timestep was fixed, in which statistics were being recalculated at each threshold value, rather than once at the end.

Ideally, I would like to move adding coordinates and distance filtering inside feature_detection_multithreshold_timestep, but at the moment we are not passing through the required info to do this. As the feature detection code is in need of refactoring, I'll leave this until after v1.6 to avoid conflicts with the switch to xarray

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[w-k-jones]

@freemansw1 are you happy to review this?

[codecov]

Codecov Report

Attention: Patch coverage is 68.96552% with 9 lines in your changes missing coverage. Please review.

Project coverage is 61.14%. Comparing base (d9f2df8) to head (bea540c).
Report is 41 commits behind head on RC_v1.5.x.

Files with missing lines	Patch %	Lines
tobac/feature_detection.py	68.96%	9 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           RC_v1.5.x     #452      +/-   ##
=============================================
+ Coverage      60.81%   61.14%   +0.32%     
=============================================
  Files             23       23              
  Lines           3527     3580      +53     
=============================================
+ Hits            2145     2189      +44     
- Misses          1382     1391       +9     

Flag	Coverage Δ
unittests	61.14% <68.96%> (+0.32%)	⬆️

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[github-actions]

Linting results by Pylint:

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[freemansw1]

Thanks, @w-k-jones . Checks out on my end.

[JuliaKukulies]

Looking good @w-k-jones! Thanks for also fixing the statistics issue. Just one thing I am wondering with regard to #449

[w-k-jones]

I have re-added the unsmoothed statistics functionally which was removed by mistake. The remaining failing tests and the formatting and one of the matrix tests which has the sporadic spectral filtering test bug. @JuliaKukulies & @freemansw1 are you happy for me to merge?

[JuliaKukulies]

Yes, ready to merge from my side! Thanks @w-k-jones !