Add NFP to ScalarPotentialField and its subclasses

[dpanici]

Adds NFP attribute to ScalarPotentialField and subclasses so that if used with save_mgrid method, the mgrid will correctly save the NFP

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_midres         |     +0.57 +/- 1.93     | +3.38e-03 +/- 1.15e-02 |  5.97e-01 +/- 9.7e-03  |  5.93e-01 +/- 6.1e-03  |
 test_build_transform_fft_highres        |     +0.12 +/- 1.09     | +1.13e-03 +/- 1.03e-02 |  9.52e-01 +/- 5.7e-03  |  9.50e-01 +/- 8.6e-03  |
 test_equilibrium_init_lowres            |     +0.79 +/- 2.06     | +2.99e-02 +/- 7.78e-02 |  3.81e+00 +/- 6.9e-02  |  3.78e+00 +/- 3.6e-02  |
 test_objective_compile_atf              |     -0.24 +/- 3.65     | -1.89e-02 +/- 2.89e-01 |  7.90e+00 +/- 1.9e-01  |  7.92e+00 +/- 2.1e-01  |
 test_objective_compute_atf              |     +0.40 +/- 1.70     | +4.15e-05 +/- 1.78e-04 |  1.05e-02 +/- 1.1e-04  |  1.05e-02 +/- 1.4e-04  |
 test_objective_jac_atf                  |     -2.14 +/- 3.38     | -4.17e-02 +/- 6.56e-02 |  1.90e+00 +/- 5.6e-02  |  1.94e+00 +/- 3.5e-02  |
 test_perturb_1                          |     +0.83 +/- 2.57     | +1.16e-01 +/- 3.60e-01 |  1.41e+01 +/- 7.4e-02  |  1.40e+01 +/- 3.5e-01  |
 test_proximal_jac_atf                   |     +0.33 +/- 0.88     | +2.67e-02 +/- 7.19e-02 |  8.20e+00 +/- 3.7e-02  |  8.18e+00 +/- 6.2e-02  |
 test_proximal_freeb_compute             |     +0.62 +/- 1.19     | +1.22e-03 +/- 2.34e-03 |  1.97e-01 +/- 2.0e-03  |  1.96e-01 +/- 1.2e-03  |
 test_solve_fixed_iter_compiled          |     -0.27 +/- 1.02     | -4.51e-02 +/- 1.72e-01 |  1.68e+01 +/- 8.6e-02  |  1.69e+01 +/- 1.5e-01  |
 test_build_transform_fft_lowres         |     +3.31 +/- 9.43     | +1.75e-02 +/- 4.98e-02 |  5.45e-01 +/- 2.5e-02  |  5.28e-01 +/- 4.3e-02  |
 test_equilibrium_init_medres            |     -1.45 +/- 8.12     | -6.18e-02 +/- 3.46e-01 |  4.21e+00 +/- 2.2e-01  |  4.27e+00 +/- 2.7e-01  |
 test_equilibrium_init_highres           |     -2.17 +/- 3.80     | -1.20e-01 +/- 2.10e-01 |  5.40e+00 +/- 7.3e-02  |  5.51e+00 +/- 2.0e-01  |
 test_objective_compile_dshape_current   |     +0.16 +/- 7.14     | +6.12e-03 +/- 2.79e-01 |  3.92e+00 +/- 2.8e-01  |  3.91e+00 +/- 3.1e-02  |
 test_objective_compute_dshape_current   |     +0.80 +/- 1.95     | +2.92e-05 +/- 7.13e-05 |  3.68e-03 +/- 5.1e-05  |  3.65e-03 +/- 5.0e-05  |
 test_objective_jac_dshape_current       |     -0.06 +/- 10.31    | -2.60e-05 +/- 4.48e-03 |  4.35e-02 +/- 3.6e-03  |  4.35e-02 +/- 2.6e-03  |
 test_perturb_2                          |     +1.15 +/- 2.54     | +2.26e-01 +/- 4.99e-01 |  1.99e+01 +/- 1.1e-01  |  1.97e+01 +/- 4.9e-01  |
 test_proximal_freeb_jac                 |     +0.67 +/- 1.61     | +5.11e-02 +/- 1.22e-01 |  7.63e+00 +/- 8.2e-02  |  7.58e+00 +/- 9.1e-02  |
 test_solve_fixed_iter                   |     +2.47 +/- 1.57     | +7.13e-01 +/- 4.54e-01 |  2.95e+01 +/- 3.6e-01  |  2.88e+01 +/- 2.7e-01  |
 test_LinearConstraintProjection_build   |     +2.22 +/- 2.87     | +5.09e-01 +/- 6.60e-01 |  2.35e+01 +/- 4.3e-01  |  2.29e+01 +/- 5.0e-01  |

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.57%. Comparing base (1d03918) to head (3e24666).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
desc/magnetic_fields/_core.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1416      +/-   ##
==========================================
- Coverage   95.58%   95.57%   -0.01%     
==========================================
  Files          96       96              
  Lines       24512    24523      +11     
==========================================
+ Hits        23429    23439      +10     
- Misses       1083     1084       +1     

Files with missing lines	Coverage Δ
desc/magnetic_fields/_dommaschk.py	97.20% <100.00%> (+0.03%)	⬆️
desc/magnetic_fields/_core.py	96.40% <81.81%> (-0.21%)	⬇️

... and 2 files with indirect coverage changes