Add initial implementation of multigrid optimization

[f0uriest]

Resolves #929

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|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     +0.03 +/- 2.78     | +1.35e-04 +/- 1.47e-02 |  5.30e-01 +/- 1.2e-02  |  5.30e-01 +/- 8.7e-03  |
 test_equilibrium_init_medres            |     +0.47 +/- 1.22     | +1.95e-02 +/- 5.06e-02 |  4.18e+00 +/- 4.5e-02  |  4.16e+00 +/- 2.2e-02  |
 test_equilibrium_init_highres           |     +0.42 +/- 1.86     | +2.29e-02 +/- 1.01e-01 |  5.45e+00 +/- 6.6e-02  |  5.43e+00 +/- 7.6e-02  |
 test_objective_compile_dshape_current   |     +1.22 +/- 4.96     | +5.00e-02 +/- 2.03e-01 |  4.14e+00 +/- 1.6e-01  |  4.09e+00 +/- 1.3e-01  |
 test_objective_compute_dshape_current   |     +2.51 +/- 2.41     | +1.29e-04 +/- 1.23e-04 |  5.25e-03 +/- 1.1e-04  |  5.12e-03 +/- 5.0e-05  |
 test_objective_jac_dshape_current       |     -0.49 +/- 7.55     | -2.07e-04 +/- 3.20e-03 |  4.21e-02 +/- 1.6e-03  |  4.23e-02 +/- 2.8e-03  |
 test_perturb_2                          |     +0.30 +/- 0.88     | +5.90e-02 +/- 1.73e-01 |  1.97e+01 +/- 5.8e-02  |  1.97e+01 +/- 1.6e-01  |
 test_proximal_freeb_jac                 |     -0.06 +/- 1.00     | -4.77e-03 +/- 7.39e-02 |  7.38e+00 +/- 6.1e-02  |  7.39e+00 +/- 4.2e-02  |
 test_solve_fixed_iter                   |     +0.15 +/- 1.97     | +4.63e-02 +/- 6.27e-01 |  3.19e+01 +/- 3.8e-01  |  3.18e+01 +/- 5.0e-01  |
 test_LinearConstraintProjection_build   |     +0.05 +/- 1.66     | +5.45e-03 +/- 1.70e-01 |  1.03e+01 +/- 1.6e-01  |  1.03e+01 +/- 5.9e-02  |
 test_build_transform_fft_midres         |     +1.10 +/- 8.12     | +6.79e-03 +/- 5.00e-02 |  6.24e-01 +/- 2.7e-02  |  6.17e-01 +/- 4.2e-02  |
 test_build_transform_fft_highres        |     +0.28 +/- 7.27     | +2.71e-03 +/- 7.15e-02 |  9.87e-01 +/- 6.2e-02  |  9.84e-01 +/- 3.6e-02  |
 test_equilibrium_init_lowres            |     +3.30 +/- 5.06     | +1.26e-01 +/- 1.93e-01 |  3.95e+00 +/- 1.6e-01  |  3.82e+00 +/- 1.1e-01  |
 test_objective_compile_atf              |     +2.94 +/- 2.36     | +2.40e-01 +/- 1.93e-01 |  8.39e+00 +/- 1.8e-01  |  8.15e+00 +/- 7.9e-02  |
 test_objective_compute_atf              |     +0.98 +/- 1.44     | +1.55e-04 +/- 2.26e-04 |  1.59e-02 +/- 1.8e-04  |  1.57e-02 +/- 1.3e-04  |
 test_objective_jac_atf                  |     +2.00 +/- 2.86     | +3.85e-02 +/- 5.50e-02 |  1.96e+00 +/- 4.6e-02  |  1.92e+00 +/- 3.0e-02  |
 test_perturb_1                          |     -2.55 +/- 2.49     | -3.77e-01 +/- 3.69e-01 |  1.44e+01 +/- 7.7e-02  |  1.48e+01 +/- 3.6e-01  |
 test_proximal_jac_atf                   |     -0.09 +/- 1.46     | -7.71e-03 +/- 1.21e-01 |  8.28e+00 +/- 1.1e-01  |  8.29e+00 +/- 5.4e-02  |
 test_proximal_freeb_compute             |     +1.63 +/- 1.44     | +3.24e-03 +/- 2.87e-03 |  2.02e-01 +/- 2.1e-03  |  1.99e-01 +/- 1.9e-03  |
 test_solve_fixed_iter_compiled          |     +2.67 +/- 1.29     | +5.41e-01 +/- 2.61e-01 |  2.08e+01 +/- 1.7e-01  |  2.02e+01 +/- 2.0e-01  |

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 3.70370% with 26 lines in your changes missing coverage. Please review.

Project coverage is 95.60%. Comparing base (48eb4d0) to head (7450720). Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
desc/optimize/optimizer.py	3.70%	26 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1554      +/-   ##
==========================================
- Coverage   95.69%   95.60%   -0.10%     
==========================================
  Files         101      101              
  Lines       25633    25660      +27     
==========================================
+ Hits        24529    24531       +2     
- Misses       1104     1129      +25     

Files with missing lines	Coverage Δ
desc/optimize/optimizer.py	86.44% <3.70%> (-10.69%)	:arrow_down:

... and 2 files with indirect coverage changes