FourierXYCurve and FourierXYCoil classes

[ddudt]

Adds a new parameterization for planar curves/coils. Instead of representing $r(\theta)$ as a Fourier series like the FourierPlanarCurve, this represents $X(\theta)$ and $Y(\theta)$ as Fourier series where $r = \sqrt{X^2 + Y^2}$.

There are some simple coil shapes like an ellipse that cannot be perfectly represented with the FourierPlanarCoil class (at finite resolution). This parameterization can represent more coil geometries, at the cost of more optimization parameters and the possibility of self-intersecting curves.

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     -4.62 +/- 3.38     | -2.65e-02 +/- 1.94e-02 |  5.47e-01 +/- 1.2e-02  |  5.73e-01 +/- 1.5e-02  |
 test_equilibrium_init_medres            |     -0.61 +/- 2.52     | -2.68e-02 +/- 1.11e-01 |  4.37e+00 +/- 8.6e-02  |  4.40e+00 +/- 7.0e-02  |
 test_equilibrium_init_highres           |     +0.21 +/- 2.00     | +1.18e-02 +/- 1.14e-01 |  5.72e+00 +/- 8.7e-02  |  5.71e+00 +/- 7.4e-02  |
 test_objective_compile_dshape_current   |     +1.18 +/- 3.59     | +4.74e-02 +/- 1.44e-01 |  4.07e+00 +/- 1.2e-01  |  4.02e+00 +/- 8.4e-02  |
 test_objective_compute_dshape_current   |     +5.31 +/- 5.91     | +2.78e-04 +/- 3.10e-04 |  5.52e-03 +/- 2.5e-04  |  5.24e-03 +/- 1.9e-04  |
 test_objective_jac_dshape_current       |     +4.78 +/- 4.66     | +2.03e-03 +/- 1.98e-03 |  4.46e-02 +/- 1.7e-03  |  4.25e-02 +/- 9.7e-04  |
 test_perturb_2                          |     +1.12 +/- 3.77     | +2.27e-01 +/- 7.67e-01 |  2.06e+01 +/- 6.7e-01  |  2.03e+01 +/- 3.7e-01  |
 test_proximal_freeb_jac                 |     -2.58 +/- 2.02     | -1.97e-01 +/- 1.55e-01 |  7.46e+00 +/- 1.1e-01  |  7.65e+00 +/- 1.1e-01  |
 test_solve_fixed_iter                   |     -1.41 +/- 3.03     | -4.73e-01 +/- 1.02e+00 |  3.31e+01 +/- 8.1e-01  |  3.36e+01 +/- 6.2e-01  |
 test_LinearConstraintProjection_build   |     +1.13 +/- 1.95     | +1.21e-01 +/- 2.08e-01 |  1.08e+01 +/- 1.5e-01  |  1.06e+01 +/- 1.4e-01  |
 test_build_transform_fft_midres         |     -3.67 +/- 2.78     | -2.39e-02 +/- 1.81e-02 |  6.26e-01 +/- 1.7e-02  |  6.50e-01 +/- 7.1e-03  |
 test_build_transform_fft_highres        |     -2.86 +/- 3.52     | -2.91e-02 +/- 3.58e-02 |  9.89e-01 +/- 3.4e-02  |  1.02e+00 +/- 9.7e-03  |
 test_equilibrium_init_lowres            |     -1.96 +/- 8.67     | -7.85e-02 +/- 3.48e-01 |  3.94e+00 +/- 1.7e-01  |  4.01e+00 +/- 3.1e-01  |
 test_objective_compile_atf              |     -2.23 +/- 3.39     | -1.86e-01 +/- 2.82e-01 |  8.15e+00 +/- 2.4e-01  |  8.34e+00 +/- 1.5e-01  |
 test_objective_compute_atf              |     -3.11 +/- 2.92     | -5.10e-04 +/- 4.79e-04 |  1.59e-02 +/- 2.4e-04  |  1.64e-02 +/- 4.1e-04  |
 test_objective_jac_atf                  |     -1.80 +/- 4.04     | -3.60e-02 +/- 8.09e-02 |  1.97e+00 +/- 7.4e-02  |  2.01e+00 +/- 3.3e-02  |
 test_perturb_1                          |     -4.64 +/- 2.27     | -7.37e-01 +/- 3.61e-01 |  1.51e+01 +/- 3.3e-01  |  1.59e+01 +/- 1.5e-01  |
 test_proximal_jac_atf                   |     -0.03 +/- 0.85     | -2.75e-03 +/- 7.08e-02 |  8.29e+00 +/- 5.7e-02  |  8.30e+00 +/- 4.2e-02  |
 test_proximal_freeb_compute             |     -1.76 +/- 0.73     | -3.53e-03 +/- 1.47e-03 |  1.97e-01 +/- 1.0e-03  |  2.01e-01 +/- 1.1e-03  |
 test_solve_fixed_iter_compiled          |     +0.07 +/- 1.71     | +1.45e-02 +/- 3.55e-01 |  2.08e+01 +/- 2.6e-01  |  2.08e+01 +/- 2.4e-01  |