Clean up TODOs and FIXMEs

[f0uriest]

Renames all FIXMEs to TODOs, and removes some old ones that are either already done or are not planned.

At the next dev/triage meeting we should go through the remaining ones and either make actual issues for them or remove them.

Resolves #1365

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_midres         |     -3.86 +/- 6.07     | -2.52e-02 +/- 3.97e-02 |  6.29e-01 +/- 3.7e-02  |  6.54e-01 +/- 1.5e-02  |
 test_build_transform_fft_highres        |     +1.33 +/- 3.06     | +1.31e-02 +/- 3.02e-02 |  1.00e+00 +/- 1.5e-02  |  9.87e-01 +/- 2.6e-02  |
 test_equilibrium_init_lowres            |     -2.41 +/- 4.99     | -1.00e-01 +/- 2.07e-01 |  4.06e+00 +/- 1.3e-01  |  4.16e+00 +/- 1.6e-01  |
 test_objective_compile_atf              |     +2.27 +/- 3.78     | +1.79e-01 +/- 2.98e-01 |  8.06e+00 +/- 1.9e-01  |  7.88e+00 +/- 2.3e-01  |
 test_objective_compute_atf              |     -0.23 +/- 4.00     | -2.47e-05 +/- 4.27e-04 |  1.07e-02 +/- 3.7e-04  |  1.07e-02 +/- 2.1e-04  |
 test_objective_jac_atf                  |     -0.88 +/- 2.96     | -1.71e-02 +/- 5.76e-02 |  1.93e+00 +/- 2.7e-02  |  1.95e+00 +/- 5.1e-02  |
 test_perturb_1                          |     +0.06 +/- 4.43     | +8.37e-03 +/- 6.44e-01 |  1.46e+01 +/- 3.1e-01  |  1.45e+01 +/- 5.7e-01  |
 test_proximal_jac_atf                   |     -0.63 +/- 1.25     | -5.17e-02 +/- 1.03e-01 |  8.19e+00 +/- 8.7e-02  |  8.24e+00 +/- 5.5e-02  |
 test_proximal_freeb_compute             |     -1.78 +/- 1.73     | -3.60e-03 +/- 3.49e-03 |  1.98e-01 +/- 1.4e-03  |  2.02e-01 +/- 3.2e-03  |
 test_build_transform_fft_lowres         |     +1.38 +/- 4.20     | +7.26e-03 +/- 2.21e-02 |  5.33e-01 +/- 2.1e-02  |  5.26e-01 +/- 7.0e-03  |
 test_equilibrium_init_medres            |     -0.42 +/- 0.88     | -1.78e-02 +/- 3.69e-02 |  4.16e+00 +/- 2.5e-02  |  4.18e+00 +/- 2.7e-02  |
 test_equilibrium_init_highres           |     -0.03 +/- 1.15     | -1.52e-03 +/- 6.24e-02 |  5.45e+00 +/- 2.6e-02  |  5.45e+00 +/- 5.7e-02  |
 test_objective_compile_dshape_current   |     +0.18 +/- 0.85     | +6.98e-03 +/- 3.29e-02 |  3.88e+00 +/- 1.9e-02  |  3.87e+00 +/- 2.7e-02  |
 test_objective_compute_dshape_current   |     -1.17 +/- 1.66     | -4.26e-05 +/- 6.08e-05 |  3.61e-03 +/- 4.0e-05  |  3.66e-03 +/- 4.6e-05  |
 test_objective_jac_dshape_current       |     -2.42 +/- 8.13     | -1.02e-03 +/- 3.43e-03 |  4.12e-02 +/- 2.2e-03  |  4.22e-02 +/- 2.6e-03  |
 test_perturb_2                          |     +0.50 +/- 1.54     | +9.46e-02 +/- 2.92e-01 |  1.91e+01 +/- 1.9e-01  |  1.90e+01 +/- 2.2e-01  |
 test_proximal_freeb_jac                 |     -0.90 +/- 1.63     | -6.79e-02 +/- 1.22e-01 |  7.45e+00 +/- 1.1e-01  |  7.52e+00 +/- 5.2e-02  |
 test_solve_fixed_iter                   |     -1.61 +/- 60.10    | -8.25e-02 +/- 3.08e+00 |  5.04e+00 +/- 2.2e+00  |  5.12e+00 +/- 2.2e+00  |

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.50%. Comparing base (ff9d414) to head (c76d5ee).
Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1359      +/-   ##
==========================================
- Coverage   95.52%   95.50%   -0.02%     
==========================================
  Files          96       96              
  Lines       24029    24028       -1     
==========================================
- Hits        22953    22948       -5     
- Misses       1076     1080       +4     

Files with missing lines	Coverage Δ
desc/backend.py	90.44% <100.00%> (ø)
desc/basis.py	98.18% <ø> (ø)
desc/batching.py	86.62% <ø> (ø)
desc/coils.py	97.47% <100.00%> (ø)
desc/compute/_geometry.py	99.02% <ø> (-0.49%)	⬇️
desc/compute/_omnigenity.py	99.37% <ø> (-0.63%)	⬇️
desc/compute/_profiles.py	99.19% <ø> (ø)
desc/compute/_stability.py	100.00% <ø> (ø)
desc/compute/_surface.py	100.00% <ø> (ø)
desc/continuation.py	93.26% <ø> (ø)
... and 29 more

---- 🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests

[YigitElma]

Good for now, we can clean-up the rest later