Methods for transforming to and from field-aligned coordinates

[johnomotani]

Includes changes in #104, which ideally should be merged before reviewing this.

Thanks to @friva000 from whose routines I lifted the fft interpolation logic.

[pep8speaks]

Hello @johnomotani! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers:

Comment last updated at 2020-03-18 11:21:43 UTC

[johnomotani]

There are lots of PEP8 complaints from xbout/tests/test_boutdataarray.py about too-long lines, but I think it's best to ignore them. Those lines are effectively a table of expected results, which are much easier to read without splitting the lines.

[ZedThree]

Might be a way to comment the region to say "ignore this bit"?

[johnomotani]

Might be a way to comment the region to say "ignore this bit"?

It's possible, but as far as I can see (https://flake8.pycqa.org/en/latest/user/violations.html) requires a comment on each line. That's not very nice, but I guess is better in the long run as it means we can run flake8 over the whole project and not get errors that we want to ignore. I'll add the suppressions.

[johnomotani]

:+1: suppressing the errors that we want to ignore showed a too-long line error that needed fixing but which I had missed in the noise before, so I think it is a good idea. Thanks @ZedThree!

[codecov-io]

Codecov Report

Merging #105 into master will increase coverage by 1.94%. The diff coverage is 79.48%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
+ Coverage   48.05%   50.00%   +1.94%     
==========================================
  Files          11       11              
  Lines        1105     1172      +67     
  Branches      227      233       +6     
==========================================
+ Hits          531      586      +55     
- Misses        506      514       +8     
- Partials       68       72       +4     

Impacted Files	Coverage Δ
xbout/geometries.py	75.25% <70.00%> (+0.81%)	:arrow_up:
xbout/boutdataset.py	66.86% <80.76%> (+1.92%)	:arrow_up:
xbout/boutdataarray.py	70.83% <80.95%> (+7.87%)	:arrow_up:

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[ZedThree]

I feel like there's probably a more compact way to write the tests, but these are nice and explicit at least!