compute the rotational transform on a magnetic axis

[andrewgiuliani]

This PR introduces functionality to compute the rotational transform on a magnetic axis, using an algorithm shared with me by @landreman and suggested by Stuart Hudson. I've put this function in util/magnetic_axis_helpers.py.

Any suggestions of addition unit tests where the analytical rotational transform is known are welcome.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.52%. Comparing base (5b1cb70) to head (5663c88).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
+ Coverage   91.43%   91.52%   +0.08%     
==========================================
  Files          73       74       +1     
  Lines       12701    12819     +118     
==========================================
+ Hits        11613    11732     +119     
+ Misses       1088     1087       -1     

Flag	Coverage Δ
unittests	91.52% <100.00%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[landreman]

What do you think about putting this function and test in simsopt.field rather than simsopt.util?

[andrewgiuliani]

What do you think about putting this function and test in simsopt.field rather than simsopt.util?

I've moved the magnetic_axis_helpers.py file to the simsopt.field directory. This file would be a good place to include a function that actually computes a field's magnetic axis, in an upcoming PR.