add Coils and magnetic fields to docs api

[dpanici]

No description provided.

[codecov]

Codecov Report

Merging #615 (3f51e10) into master (6adcb4c) will increase coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #615      +/-   ##
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+ Coverage   94.30%   94.32%   +0.01%     
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  Files          77       77              
  Lines       17696    17696              
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+ Hits        16689    16691       +2     
+ Misses       1007     1005       -2     

Files Changed	Coverage Δ
desc/coils.py	98.83% <ø> (ø)

... and 3 files with indirect coverage changes

[dpanici]

black format the doc code

[ddudt]

update example code to have black formatting

[f0uriest]

FYI, I'm not sure about having grid objects assigned to each coil. I was considering getting rid of it in #583 but felt we should discuss it first. I think it would be better to add something like source_grid as an optional argument to compute_magnetic_field, especially since after #583 whatever grid you assign to the coil won't be used by the underlying Curve.compute method.

[dpanici]

I agree, that is something we can change in #494 when I update the coil stuff to reflect the #583 changes. This PR though is just for the docs