Fix definition of b0 in vmec output

[f0uriest]

The quantity b0 output by VMEC is defined as $\langle R B \rangle / \langle R \rangle$ where $\langle ... \rangle$ is a flux surface average. We currently compute it as

DESC/desc/vmec.py

Lines 488 to 491 in 90d1ecb

	b0 = file.createVariable("b0", np.float64)
	b0.long_name = "average B_tor on axis"
	b0.units = "T"
	b0[:] = data_axis["G"][0] / data_axis["R"][0]

This is fine in axisymmetry, but for stellarators we're using the major radius at phi=0 rather than the average.

This PR changes it to use the equivalent definition, b0 = data_axis['<|B|>'] which avoids needing to deal with zero jacobians in the surface averages in the VMECIO part of things

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.94%. Comparing base (90d1ecb) to head (1675e12).
Report is 1542 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1012   +/-   ##
=======================================
  Coverage   94.94%   94.94%           
=======================================
  Files          87       87           
  Lines       21884    21884           
=======================================
  Hits        20777    20777           
  Misses       1107     1107           

Files with missing lines	Coverage Δ
desc/vmec.py	92.21% <100.00%> (ø)

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     +1.23 +/- 4.17     | +6.44e-03 +/- 2.18e-02 |  5.28e-01 +/- 2.0e-02  |  5.22e-01 +/- 8.4e-03  |
 test_build_transform_fft_midres         |     +0.56 +/- 1.93     | +3.35e-03 +/- 1.15e-02 |  5.99e-01 +/- 8.8e-03  |  5.96e-01 +/- 7.4e-03  |
 test_build_transform_fft_highres        |     -0.01 +/- 2.28     | -9.92e-05 +/- 2.26e-02 |  9.91e-01 +/- 1.8e-02  |  9.91e-01 +/- 1.4e-02  |
 test_equilibrium_init_lowres            |     +1.25 +/- 4.00     | +5.09e-02 +/- 1.63e-01 |  4.12e+00 +/- 1.3e-01  |  4.07e+00 +/- 1.0e-01  |
 test_equilibrium_init_medres            |     -0.40 +/- 2.69     | -1.84e-02 +/- 1.24e-01 |  4.61e+00 +/- 6.4e-02  |  4.63e+00 +/- 1.1e-01  |
 test_equilibrium_init_highres           |     +0.99 +/- 3.00     | +6.28e-02 +/- 1.89e-01 |  6.38e+00 +/- 1.7e-01  |  6.32e+00 +/- 8.3e-02  |
 test_objective_compile_dshape_current   |     -1.55 +/- 2.86     | -5.73e-02 +/- 1.05e-01 |  3.63e+00 +/- 7.7e-02  |  3.69e+00 +/- 7.2e-02  |
 test_objective_compile_atf              |     -1.21 +/- 4.22     | -8.65e-02 +/- 3.03e-01 |  7.08e+00 +/- 1.7e-01  |  7.17e+00 +/- 2.5e-01  |
 test_objective_compute_dshape_current   |     -0.26 +/- 2.72     | -1.01e-05 +/- 1.06e-04 |  3.90e-03 +/- 5.8e-05  |  3.91e-03 +/- 8.9e-05  |
 test_objective_compute_atf              |     -2.72 +/- 3.01     | -4.84e-04 +/- 5.35e-04 |  1.73e-02 +/- 3.8e-04  |  1.78e-02 +/- 3.8e-04  |
 test_objective_jac_dshape_current       |     -4.61 +/- 7.91     | -1.96e-03 +/- 3.36e-03 |  4.05e-02 +/- 2.0e-03  |  4.24e-02 +/- 2.7e-03  |
 test_objective_jac_atf                  |     -0.05 +/- 2.94     | -1.01e-03 +/- 5.57e-02 |  1.89e+00 +/- 4.7e-02  |  1.89e+00 +/- 3.0e-02  |
 test_perturb_1                          |     -2.60 +/- 3.44     | -3.07e-01 +/- 4.05e-01 |  1.15e+01 +/- 3.6e-01  |  1.18e+01 +/- 1.8e-01  |
 test_perturb_2                          |     +0.37 +/- 2.75     | +6.12e-02 +/- 4.51e-01 |  1.64e+01 +/- 4.2e-01  |  1.64e+01 +/- 1.7e-01  |
 test_proximal_jac_atf                   |     +1.81 +/- 1.40     | +1.27e-01 +/- 9.82e-02 |  7.15e+00 +/- 5.9e-02  |  7.02e+00 +/- 7.9e-02  |
 test_proximal_freeb_compute             |     +0.27 +/- 0.81     | +3.54e-04 +/- 1.04e-03 |  1.29e-01 +/- 6.0e-04  |  1.29e-01 +/- 8.5e-04  |
 test_proximal_freeb_jac                 |     -0.52 +/- 1.63     | -3.79e-02 +/- 1.19e-01 |  7.24e+00 +/- 9.4e-02  |  7.28e+00 +/- 7.3e-02  |