VMEC wout fixes

[ddudt]

1. Resolves Sign difference in buco when writing a vmec input file vs writing a vmec wout file #915 by flipping the sign of buco
2. Also flips the sign of jcuru (same issue as above)
3. Adds the quantity vp, which is dV/ds / 4pi^2 as documented here
4. Adds the quantity nextcur for the number of coils, which is hard-coded to 0 for fixed-boundary solutions
5. Refactors some of the scalar computations to make it more efficient and save compute time

VMECIO.save assumes the equilibrium was a fixed-boundary solution (we hard-code some outputs to their fixed-boundary values). But now that we have free-boundary working we could think about accommodating those free-boundary values (in a future PR). I think we would need to pass in the MagneticField that was used in addition to the Equilibrium.

[codecov]

Codecov Report

Attention: Patch coverage is 99.17355% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 94.95%. Comparing base (0d72aa0) to head (af35c49).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #925      +/-   ##
==========================================
- Coverage   94.96%   94.95%   -0.01%     
==========================================
  Files          83       83              
  Lines       21005    21006       +1     
==========================================
  Hits        19947    19947              
- Misses       1058     1059       +1     

Files	Coverage Δ
desc/magnetic_fields.py	96.91% <100.00%> (ø)
desc/vmec.py	92.21% <99.16%> (-0.10%)	⬇️

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     +2.36 +/- 13.09    | +2.17e-04 +/- 1.21e-03 |  9.44e-03 +/- 1.2e-03  |  9.22e-03 +/- 6.3e-05  |
 test_build_transform_fft_midres         |     -1.86 +/- 1.82     | -1.67e-03 +/- 1.64e-03 |  8.84e-02 +/- 1.1e-03  |  9.01e-02 +/- 1.2e-03  |
 test_build_transform_fft_highres        |     -0.62 +/- 1.08     | -2.88e-03 +/- 5.04e-03 |  4.62e-01 +/- 3.5e-03  |  4.65e-01 +/- 3.6e-03  |
 test_equilibrium_init_lowres            |     -0.08 +/- 2.71     | -2.23e-04 +/- 7.35e-03 |  2.71e-01 +/- 4.7e-03  |  2.72e-01 +/- 5.6e-03  |
 test_equilibrium_init_medres            |     -2.50 +/- 1.98     | -1.59e-02 +/- 1.25e-02 |  6.18e-01 +/- 7.7e-03  |  6.34e-01 +/- 9.9e-03  |
 test_equilibrium_init_highres           |     -0.63 +/- 1.76     | -1.41e-02 +/- 3.97e-02 |  2.24e+00 +/- 2.7e-02  |  2.25e+00 +/- 2.9e-02  |
 test_objective_compile_dshape_current   |     -1.44 +/- 9.73     | -5.93e-02 +/- 4.00e-01 |  4.05e+00 +/- 2.7e-01  |  4.11e+00 +/- 2.9e-01  |
 test_objective_compile_atf              |     +0.41 +/- 6.07     | +3.41e-02 +/- 5.00e-01 |  8.27e+00 +/- 3.1e-01  |  8.24e+00 +/- 3.9e-01  |
 test_objective_compute_dshape_current   |     -1.18 +/- 4.00     | -6.32e-05 +/- 2.15e-04 |  5.31e-03 +/- 1.4e-04  |  5.38e-03 +/- 1.6e-04  |
 test_objective_compute_atf              |     -0.23 +/- 6.29     | -2.85e-05 +/- 7.65e-04 |  1.21e-02 +/- 5.2e-04  |  1.22e-02 +/- 5.6e-04  |
 test_objective_jac_dshape_current       |     +3.10 +/- 8.33     | +1.46e-03 +/- 3.92e-03 |  4.86e-02 +/- 2.8e-03  |  4.71e-02 +/- 2.8e-03  |
 test_objective_jac_atf                  |     -0.39 +/- 4.59     | -9.28e-03 +/- 1.11e-01 |  2.40e+00 +/- 7.7e-02  |  2.41e+00 +/- 8.0e-02  |
 test_perturb_1                          |     +3.72 +/- 4.29     | +5.87e-01 +/- 6.76e-01 |  1.64e+01 +/- 4.8e-01  |  1.58e+01 +/- 4.8e-01  |
 test_perturb_2                          |     +1.92 +/- 4.30     | +4.12e-01 +/- 9.24e-01 |  2.19e+01 +/- 3.0e-01  |  2.15e+01 +/- 8.7e-01  |
 test_proximal_jac_atf                   |     +0.71 +/- 1.98     | +5.34e-02 +/- 1.49e-01 |  7.62e+00 +/- 1.3e-01  |  7.57e+00 +/- 7.3e-02  |
 test_proximal_freeb_compute             |     +0.08 +/- 1.27     | +1.09e-04 +/- 1.63e-03 |  1.28e-01 +/- 1.2e-03  |  1.28e-01 +/- 1.1e-03  |
 test_proximal_freeb_jac                 |     -2.26 +/- 1.02     | -1.70e-01 +/- 7.68e-02 |  7.34e+00 +/- 5.1e-02  |  7.51e+00 +/- 5.7e-02  |

[f0uriest]

See comment about combining duplicate calculations on half/full grid

[dpanici]

I thought that we have axis limits implemented for most things?

[ddudt]

I think this was left over from before we implemented the axis limits. Updated now, thanks.