Rotating ellipse surface

[landreman]

This PR adds a convenience function to make SurfaceRZFourier surfaces that are rotating ellipses.

I decided not to make this a @classmethod so users have the option to specify either quadpoints_theta and quadpoints_phi, or specify range, nphi, and ntheta with @classmethod from_nphi_ntheta().

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.56%. Comparing base (7d88bd9) to head (b0bb926).
Report is 3 commits behind head on master.

❗ Current head b0bb926 differs from pull request most recent head bc79d97. Consider uploading reports for the commit bc79d97 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   91.10%   91.56%   +0.46%     
==========================================
  Files          74       74              
  Lines       12956    12923      -33     
==========================================
+ Hits        11803    11833      +30     
+ Misses       1153     1090      -63     

Flag	Coverage Δ
unittests	91.56% <100.00%> (+0.46%)	⬆️

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[rogeriojorge]

Looks good to me, just a comment on the possibility of an extra iota test.