Clean up API for free boundary with surface current

[f0uriest]

Resolves #869

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     +1.68 +/- 32.53    | +2.12e-04 +/- 4.09e-03 |  1.28e-02 +/- 4.1e-03  |  1.26e-02 +/- 1.2e-04  |
 test_build_transform_fft_midres         |     +3.01 +/- 1.13     | +2.76e-03 +/- 1.04e-03 |  9.44e-02 +/- 8.9e-04  |  9.16e-02 +/- 5.4e-04  |
-test_build_transform_fft_highres        |     +2.67 +/- 0.69     | +1.23e-02 +/- 3.17e-03 |  4.73e-01 +/- 2.1e-03  |  4.60e-01 +/- 2.3e-03  |
 test_equilibrium_init_lowres            |     +0.49 +/- 0.82     | +2.01e-03 +/- 3.38e-03 |  4.12e-01 +/- 2.0e-03  |  4.10e-01 +/- 2.7e-03  |
 test_equilibrium_init_medres            |     +1.05 +/- 0.98     | +7.98e-03 +/- 7.50e-03 |  7.70e-01 +/- 6.4e-03  |  7.62e-01 +/- 3.8e-03  |
 test_equilibrium_init_highres           |     +1.79 +/- 1.24     | +4.14e-02 +/- 2.86e-02 |  2.36e+00 +/- 1.6e-02  |  2.32e+00 +/- 2.4e-02  |
 test_objective_compile_dshape_current   |     +1.61 +/- 0.54     | +7.43e-02 +/- 2.49e-02 |  4.68e+00 +/- 2.1e-02  |  4.60e+00 +/- 1.4e-02  |
 test_objective_compile_atf              |     +1.02 +/- 2.02     | +9.42e-02 +/- 1.85e-01 |  9.29e+00 +/- 1.4e-01  |  9.19e+00 +/- 1.3e-01  |
 test_objective_compute_dshape_current   |     +3.07 +/- 4.48     | +2.13e-04 +/- 3.11e-04 |  7.17e-03 +/- 9.3e-05  |  6.95e-03 +/- 3.0e-04  |
 test_objective_compute_atf              |     +0.20 +/- 5.19     | +2.89e-05 +/- 7.32e-04 |  1.42e-02 +/- 5.1e-04  |  1.41e-02 +/- 5.3e-04  |
 test_objective_jac_dshape_current       |     +1.11 +/- 6.19     | +5.22e-04 +/- 2.91e-03 |  4.76e-02 +/- 2.0e-03  |  4.71e-02 +/- 2.1e-03  |
 test_objective_jac_atf                  |     +0.12 +/- 5.00     | +2.45e-03 +/- 1.02e-01 |  2.03e+00 +/- 7.9e-02  |  2.03e+00 +/- 6.4e-02  |
 test_perturb_1                          |     +0.84 +/- 0.54     | +1.14e-01 +/- 7.33e-02 |  1.36e+01 +/- 4.4e-02  |  1.35e+01 +/- 5.8e-02  |
 test_perturb_2                          |     +1.24 +/- 0.66     | +2.40e-01 +/- 1.29e-01 |  1.97e+01 +/- 1.1e-01  |  1.94e+01 +/- 6.8e-02  |
 test_proximal_jac_atf                   |     -0.32 +/- 1.04     | -2.20e-02 +/- 7.07e-02 |  6.76e+00 +/- 5.7e-02  |  6.79e+00 +/- 4.3e-02  |
-test_proximal_freeb_compute             |     +4.79 +/- 0.63     | +6.00e-03 +/- 7.88e-04 |  1.31e-01 +/- 5.3e-04  |  1.25e-01 +/- 5.8e-04  |
 test_proximal_freeb_jac                 |     +0.48 +/- 1.04     | +3.88e-02 +/- 8.39e-02 |  8.10e+00 +/- 6.8e-02  |  8.06e+00 +/- 5.0e-02  |

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.10%. Comparing base (43f354e) to head (fc02b0a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #894   +/-   ##
=======================================
  Coverage   95.10%   95.10%           
=======================================
  Files          83       83           
  Lines       20986    21003   +17     
=======================================
+ Hits        19958    19975   +17     
  Misses       1028     1028           

Files	Coverage Δ
desc/equilibrium/equilibrium.py	96.59% <100.00%> (+0.11%)	⬆️
desc/objectives/_free_boundary.py	96.80% <100.00%> (-0.08%)	⬇️

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[dpanici]

I think that adding a M_Phi and N_Phi to the current potenital from_surface method makes sense, kind of an oversight on my part to not have that as an option. If you want to add it to this PR I think it would fit or I can commit it today to this as well

[dpanici]

• small comment on docstring of the new eq properties accessing the surface current field