C0 fourier

[kianorr]

Resolves #1122

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_midres         |     +2.45 +/- 4.82     | +1.47e-02 +/- 2.88e-02 |  6.13e-01 +/- 2.8e-02  |  5.98e-01 +/- 6.9e-03  |
 test_build_transform_fft_highres        |     +1.15 +/- 3.49     | +1.13e-02 +/- 3.43e-02 |  9.94e-01 +/- 2.9e-02  |  9.83e-01 +/- 1.9e-02  |
 test_equilibrium_init_lowres            |     +2.18 +/- 2.34     | +8.15e-02 +/- 8.77e-02 |  3.82e+00 +/- 8.5e-02  |  3.74e+00 +/- 2.2e-02  |
 test_objective_compile_atf              |     +0.49 +/- 3.78     | +3.82e-02 +/- 2.97e-01 |  7.90e+00 +/- 2.3e-01  |  7.87e+00 +/- 1.9e-01  |
 test_objective_compute_atf              |     +1.87 +/- 2.39     | +1.94e-04 +/- 2.48e-04 |  1.06e-02 +/- 2.3e-04  |  1.04e-02 +/- 8.3e-05  |
 test_objective_jac_atf                  |     -0.11 +/- 2.94     | -2.22e-03 +/- 5.68e-02 |  1.93e+00 +/- 3.3e-02  |  1.93e+00 +/- 4.7e-02  |
 test_perturb_1                          |     -0.31 +/- 2.48     | -3.96e-02 +/- 3.18e-01 |  1.28e+01 +/- 2.1e-01  |  1.28e+01 +/- 2.4e-01  |
 test_proximal_jac_atf                   |     +0.91 +/- 0.90     | +7.35e-02 +/- 7.29e-02 |  8.15e+00 +/- 5.3e-02  |  8.07e+00 +/- 5.0e-02  |
 test_proximal_freeb_compute             |     +0.26 +/- 0.98     | +4.66e-04 +/- 1.77e-03 |  1.82e-01 +/- 1.6e-03  |  1.81e-01 +/- 6.7e-04  |
 test_build_transform_fft_lowres         |     +0.75 +/- 3.88     | +4.33e-03 +/- 2.23e-02 |  5.79e-01 +/- 1.2e-02  |  5.74e-01 +/- 1.9e-02  |
 test_equilibrium_init_medres            |     +0.45 +/- 4.43     | +2.05e-02 +/- 2.01e-01 |  4.57e+00 +/- 1.6e-01  |  4.55e+00 +/- 1.2e-01  |
 test_equilibrium_init_highres           |     +0.08 +/- 3.25     | +4.81e-03 +/- 1.93e-01 |  5.95e+00 +/- 1.1e-01  |  5.95e+00 +/- 1.6e-01  |
 test_objective_compile_dshape_current   |     -1.46 +/- 3.21     | -5.93e-02 +/- 1.31e-01 |  4.00e+00 +/- 7.4e-02  |  4.06e+00 +/- 1.1e-01  |
 test_objective_compute_dshape_current   |     -0.04 +/- 3.72     | -1.44e-06 +/- 1.37e-04 |  3.68e-03 +/- 1.1e-04  |  3.68e-03 +/- 8.3e-05  |
 test_objective_jac_dshape_current       |     -0.62 +/- 5.51     | -2.61e-04 +/- 2.33e-03 |  4.20e-02 +/- 1.4e-03  |  4.22e-02 +/- 1.8e-03  |
 test_perturb_2                          |     +1.06 +/- 3.58     | +2.01e-01 +/- 6.79e-01 |  1.92e+01 +/- 4.8e-01  |  1.90e+01 +/- 4.8e-01  |
 test_proximal_freeb_jac                 |     +0.15 +/- 2.06     | +1.14e-02 +/- 1.59e-01 |  7.74e+00 +/- 1.0e-01  |  7.73e+00 +/- 1.2e-01  |
 test_solve_fixed_iter                   |     +0.01 +/- 61.52    | +4.80e-04 +/- 3.08e+00 |  5.01e+00 +/- 2.2e+00  |  5.00e+00 +/- 2.2e+00  |

[kianorr]

I get this after optimizing with a length target of 2pi: