Mgrid improvements

[aaroncbader]

This PR updates the mgrid writing script to also include the arrays for the Magnetic Potential. These values are needed in some codes (such as BMW).

There is a separate issue in the mgrid to allow for calculation of multiple coil sets separately with scaled currents, but that requires some more thought.

[mbkumar]

In the MGrid class, the method from_file should read the A_vec data as well.

[landreman]

I've never seen mgrid files that contain the vector potential before. Is there some version of xgrid other than the one in stellopt that writes mgrid files containing A? mgrid files are already quite large and storing A makes them twice as big, and vmec doesn't need A, so I'm not sure it makes sense to always include A.

[aaroncbader]

I've never seen mgrid files that contain the vector potential before. Is there some version of xgrid other than the one in stellopt that writes mgrid files containing A? mgrid files are already quite large and storing A makes them twice as big, and vmec doesn't need A, so I'm not sure it makes sense to always include A.

The oak ridge version of MakeGrid includes the vector potential. It is necessary to run BMW.

[aaroncbader]

Will make including potential optional and default to off

[aaroncbader]

I would like to update the test that reads an mgrid file to include one with the potential fields enabled. But I don't know how to regenerate the test mgrid file.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 91.17%. Comparing base (1fb6e87) to head (377a793).

Files	Patch %	Lines
src/simsopt/field/mgrid.py	85.48%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #396 +/- ## ========================================== - Coverage 91.56% 91.17% -0.39% ========================================== Files 74 74 Lines 12762 12829 +67 ========================================== + Hits 11685 11697 +12 - Misses 1077 1132 +55 ``` | [Flag](https://app.codecov.io/gh/hiddenSymmetries/simsopt/pull/396/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hiddenSymmetries) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/hiddenSymmetries/simsopt/pull/396/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hiddenSymmetries) | `91.17% <87.50%> (-0.39%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hiddenSymmetries#carryforward-flags-in-the-pull-request-comment) to find out more.

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[landreman]

It looks like that test file mgrid.pnas-qa-test-lowres-standard.nc was added by Tony Qian, so you could ask him. Or just use a different file for testing mgrid files that contain A.

[aaroncbader]

Should I be worried about the failed tests?

[mbkumar]

@aaroncbader The issue is not with your tests, but one of Spec tests is failing. I am not sure if that test is using the mgrid file that was modified, but I doubt it.

[aaroncbader]

There looks to be an issue with loading mgrids. But it's not one that I introduced. Simsopt is unable to load mgrids that it saves. It looks like this problem predates my changes. I'll see if I can fix it.

[landreman]

Should I be worried about the failed tests?

No need to worry - these failures are a known problem, #357, which is independent of this PR. @abaillod @smiet Any hope of fixing #357?

[aaroncbader]

ok, I've fixed up the mgrid from_file function and added a test for a new mgrid with a potential value