Add Vector Potential Calculation to `Coil` classes and Most `MagneticField` Classes

[dpanici]

Add utilities for calculating the magnetic vector potential for fields and coils/currents (assuming the Coulomb gauge). These will be useful for reconstruction as the most efficient way to calculate flux through a diagnostic flux loop is to integrate the vector potential. The utilities here should allow for calculating the vector potential from most sources of interest: coils and the plasma contribution (through K=nxB and the general biot-savart law here, as the loops are located outside from the plasma)

• [x] Add general biot savart law for vector potential
• [x] Add Hanson-Hirhsman biot savart law for coil vector potential
• [x] Add quadrature biot savart law for coil vector potential
• [x] add to current potential
• [x] add tests for current potential (tests loop integral against enclosed toroidal flux)
• [x] add to coils
• [x] add test for coils for simple loop on z-axis
• [x] add a test against enclosed toroidal flux doing a loop integral (like the one in this paper perhaps)
  • [x] understand why flipping orientation of the coil parameterization does not flip sign of A? Check signs of A and x_s, though even just the A does not change sign as I expect... but I'd expect xs to change sign when the parameterizwion is flipped
• [x] add to the analytic fields
• [x] add tests for the analytic fields
• [x] Add VectorPotentialField
• [x] Add to SplineMagneticField (from mgrid)
• [x] note all gauges (try to put Coulomb Gauge for all) TF and VF are, and the rest are either from AD or mgrid, so we can't really check that
• [x] Use Vector potential for ToroidalFlux objective (possibly as an option, since not all B fields have a vector potential field)

Resolves #1023

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.21429% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.44%. Comparing base (d2e9a2c) to head (98053b3). Report is 1484 commits behind head on master.

Files with missing lines	Patch %	Lines
desc/magnetic_fields/_core.py	97.05%	5 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1026      +/-   ##
==========================================
+ Coverage   95.41%   95.44%   +0.02%     
==========================================
  Files          95       95              
  Lines       23186    23406     +220     
==========================================
+ Hits        22124    22340     +216     
- Misses       1062     1066       +4     

Files with missing lines	Coverage Δ
desc/coils.py	97.46% <100.00%> (+0.21%)	:arrow_up:
desc/magnetic_fields/__init__.py	100.00% <ø> (ø)
desc/magnetic_fields/_current_potential.py	99.44% <100.00%> (+0.03%)	:arrow_up:
desc/objectives/_coils.py	99.17% <100.00%> (+0.03%)	:arrow_up:
desc/magnetic_fields/_core.py	96.56% <97.05%> (+0.16%)	:arrow_up:

[github-actions[bot]]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     +0.27 +/- 10.81    | +1.49e-03 +/- 5.90e-02 |  5.47e-01 +/- 5.4e-02  |  5.46e-01 +/- 2.5e-02  |
 test_equilibrium_init_medres            |     -0.99 +/- 4.81     | -4.27e-02 +/- 2.08e-01 |  4.29e+00 +/- 2.0e-01  |  4.33e+00 +/- 5.2e-02  |
 test_equilibrium_init_highres           |     +1.52 +/- 1.92     | +8.65e-02 +/- 1.10e-01 |  5.79e+00 +/- 7.8e-02  |  5.71e+00 +/- 7.7e-02  |
 test_objective_compile_dshape_current   |     +0.62 +/- 1.14     | +2.45e-02 +/- 4.47e-02 |  3.94e+00 +/- 3.7e-02  |  3.92e+00 +/- 2.5e-02  |
 test_objective_compute_dshape_current   |     -0.40 +/- 3.89     | -1.43e-05 +/- 1.38e-04 |  3.53e-03 +/- 8.2e-05  |  3.54e-03 +/- 1.1e-04  |
 test_objective_jac_dshape_current       |     +2.73 +/- 8.20     | +1.13e-03 +/- 3.40e-03 |  4.26e-02 +/- 3.1e-03  |  4.15e-02 +/- 1.5e-03  |
 test_perturb_2                          |     +0.75 +/- 1.27     | +1.36e-01 +/- 2.28e-01 |  1.82e+01 +/- 1.5e-01  |  1.80e+01 +/- 1.7e-01  |
 test_proximal_freeb_jac                 |     +1.00 +/- 1.60     | +7.50e-02 +/- 1.20e-01 |  7.61e+00 +/- 8.3e-02  |  7.53e+00 +/- 8.7e-02  |
 test_solve_fixed_iter                   |     +0.98 +/- 60.04    | +4.91e-02 +/- 3.01e+00 |  5.06e+00 +/- 2.1e+00  |  5.01e+00 +/- 2.2e+00  |

[f0uriest]

div/grad/curl in jax (assumes cartesian coordinates, will need some extra stuff to work in cylindrical)

def grad(fun, argnum):
    return jax.jacfwd(fun, argnum)

def div(fun, argnum):
    def divf(*args, **kwargs):
        return jnp.trace(jax.jacfwd(fun, argnum)(*args, **kwargs))
    return divf

def curl(fun, argnum):
    def curlf(*args, **kwargs):
        J = jax.jacfwd(fun, argnum)(*args, **kwargs)
        return jnp.array([J[2,1]-J[1,2],
                         J[0,2]-J[2,0],
                         J[1,0]-J[0,1]])
    return curlf

[dpanici]

remove _ fxn for toroidal field and vertical field and poloidal field