Allow QuadraticFlux etc. to take multiple MagneticField objects

[ddudt]

Resolves #1164

This allows for optimization with multiple different types of magnetic fields in combination; for example, a MixedCoilSet, a FourierCurrentPotentialField, and a VerticalMagneticField. Objectives like QuadraticFlux need to know about the total magnetic field from all of these sources, and you could combine them all in a SumMagneticField to make that work. However, that then breaks the logic of objectives that expect a particular type of magnetic field, such as the coil objectives. Rather than editing the logic of all other objectives to work with a SumMagneticField, this PR edits the logic of QuadraticFlux to take a list of fields and internally combines them into a SumMagneticField to get the total field.

TODO:

• QuadraticFlux
• BoundaryError
• other objectives that need the total magnetic field?
• tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.42%. Comparing base (b1900b3) to head (aaed44a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1205   +/-   ##
=======================================
  Coverage   95.42%   95.42%           
=======================================
  Files          89       89           
  Lines       22434    22434           
=======================================
+ Hits        21407    21408    +1     
+ Misses       1027     1026    -1     

Files	Coverage Δ
desc/objectives/_coils.py	99.14% <100.00%> (ø)

... and 2 files with indirect coverage changes

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     -0.81 +/- 7.27     | -4.42e-03 +/- 3.97e-02 |  5.42e-01 +/- 3.6e-02  |  5.46e-01 +/- 1.7e-02  |
 test_build_transform_fft_midres         |     -2.60 +/- 3.37     | -1.65e-02 +/- 2.14e-02 |  6.17e-01 +/- 1.3e-02  |  6.34e-01 +/- 1.7e-02  |
 test_build_transform_fft_highres        |     -2.03 +/- 2.81     | -2.10e-02 +/- 2.91e-02 |  1.01e+00 +/- 2.3e-02  |  1.03e+00 +/- 1.8e-02  |
 test_equilibrium_init_lowres            |     -0.10 +/- 4.94     | -4.02e-03 +/- 1.98e-01 |  4.00e+00 +/- 1.3e-01  |  4.00e+00 +/- 1.5e-01  |
 test_equilibrium_init_medres            |     +0.14 +/- 1.46     | +6.34e-03 +/- 6.54e-02 |  4.49e+00 +/- 5.6e-02  |  4.49e+00 +/- 3.3e-02  |
 test_equilibrium_init_highres           |     -0.36 +/- 2.57     | -2.16e-02 +/- 1.52e-01 |  5.90e+00 +/- 1.3e-01  |  5.92e+00 +/- 7.2e-02  |
 test_objective_compile_dshape_current   |     +0.31 +/- 0.82     | +1.21e-02 +/- 3.23e-02 |  3.93e+00 +/- 2.7e-02  |  3.92e+00 +/- 1.7e-02  |
 test_objective_compile_atf              |     -0.15 +/- 1.09     | -1.20e-02 +/- 8.73e-02 |  8.02e+00 +/- 7.0e-02  |  8.03e+00 +/- 5.2e-02  |
 test_objective_compute_dshape_current   |     -0.93 +/- 2.50     | -3.32e-05 +/- 8.93e-05 |  3.53e-03 +/- 5.0e-05  |  3.56e-03 +/- 7.4e-05  |
 test_objective_compute_atf              |     -0.64 +/- 2.93     | -6.79e-05 +/- 3.11e-04 |  1.05e-02 +/- 2.4e-04  |  1.06e-02 +/- 2.0e-04  |
 test_objective_jac_dshape_current       |     +0.01 +/- 8.15     | +5.94e-06 +/- 3.37e-03 |  4.14e-02 +/- 2.6e-03  |  4.14e-02 +/- 2.2e-03  |
 test_objective_jac_atf                  |     +2.28 +/- 2.48     | +4.37e-02 +/- 4.74e-02 |  1.96e+00 +/- 1.4e-02  |  1.91e+00 +/- 4.5e-02  |
 test_perturb_1                          |     -0.48 +/- 1.83     | -6.22e-02 +/- 2.38e-01 |  1.29e+01 +/- 1.6e-01  |  1.30e+01 +/- 1.8e-01  |
 test_perturb_2                          |     -0.75 +/- 0.84     | -1.37e-01 +/- 1.52e-01 |  1.81e+01 +/- 1.4e-01  |  1.82e+01 +/- 7.0e-02  |
 test_proximal_jac_atf                   |     -0.33 +/- 1.06     | -2.76e-02 +/- 8.74e-02 |  8.23e+00 +/- 3.8e-02  |  8.26e+00 +/- 7.9e-02  |
 test_proximal_freeb_compute             |     -1.24 +/- 0.90     | -2.28e-03 +/- 1.65e-03 |  1.82e-01 +/- 9.0e-04  |  1.84e-01 +/- 1.4e-03  |
 test_proximal_freeb_jac                 |     +0.21 +/- 1.43     | +1.54e-02 +/- 1.08e-01 |  7.55e+00 +/- 7.3e-02  |  7.53e+00 +/- 8.0e-02  |
 test_solve_fixed_iter                   |     -1.11 +/- 60.18    | -5.71e-02 +/- 3.09e+00 |  5.08e+00 +/- 2.2e+00  |  5.13e+00 +/- 2.2e+00  |