Throw error for when `Curve` is used for coil objectives

[kianorr]

Resolves #1020

The given example in #1020 uses a FourierPlanarCurve which actually results in a list of numpy arrays being created after going through tree_flatten. This PR gives a better error message, but I'm not sure if we can explicitly say that a Curve was inputted.

If there is a Curve object in a CoilSet then there are already checks in the CoilSet class that throw an error.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.33%. Comparing base (301a894) to head (f438cef). Report is 1979 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1141   +/-   ##
=======================================
  Coverage   95.33%   95.33%           
=======================================
  Files          90       90           
  Lines       22700    22702    +2     
=======================================
+ Hits        21641    21644    +3     
+ Misses       1059     1058    -1     

Files with missing lines	Coverage Δ
desc/objectives/_coils.py	99.14% <100.00%> (+<0.01%)	:arrow_up:

... and 1 file with indirect coverage changes

[github-actions[bot]]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     -1.33 +/- 7.00     | -7.30e-03 +/- 3.85e-02 |  5.43e-01 +/- 2.2e-02  |  5.50e-01 +/- 3.2e-02  |
 test_build_transform_fft_midres         |     +0.45 +/- 5.62     | +2.85e-03 +/- 3.52e-02 |  6.30e-01 +/- 1.9e-02  |  6.27e-01 +/- 3.0e-02  |
 test_build_transform_fft_highres        |     +0.71 +/- 3.26     | +7.17e-03 +/- 3.29e-02 |  1.02e+00 +/- 1.5e-02  |  1.01e+00 +/- 2.9e-02  |
 test_equilibrium_init_lowres            |     +2.27 +/- 3.82     | +8.95e-02 +/- 1.50e-01 |  4.03e+00 +/- 8.8e-02  |  3.94e+00 +/- 1.2e-01  |
 test_equilibrium_init_medres            |     +2.26 +/- 4.02     | +9.57e-02 +/- 1.70e-01 |  4.33e+00 +/- 1.4e-01  |  4.23e+00 +/- 9.2e-02  |
 test_equilibrium_init_highres           |     +0.96 +/- 2.55     | +5.35e-02 +/- 1.42e-01 |  5.63e+00 +/- 9.5e-02  |  5.58e+00 +/- 1.1e-01  |
 test_objective_compile_dshape_current   |     +0.16 +/- 2.27     | +6.33e-03 +/- 8.89e-02 |  3.93e+00 +/- 7.4e-02  |  3.92e+00 +/- 4.9e-02  |
 test_objective_compile_atf              |     +1.80 +/- 1.47     | +1.44e-01 +/- 1.18e-01 |  8.17e+00 +/- 8.8e-02  |  8.03e+00 +/- 7.8e-02  |
 test_objective_compute_dshape_current   |     -0.97 +/- 2.20     | -3.37e-05 +/- 7.66e-05 |  3.45e-03 +/- 5.4e-05  |  3.49e-03 +/- 5.5e-05  |
 test_objective_compute_atf              |     +0.59 +/- 5.83     | +6.18e-05 +/- 6.06e-04 |  1.05e-02 +/- 2.9e-04  |  1.04e-02 +/- 5.3e-04  |
 test_objective_jac_dshape_current       |     +3.46 +/- 7.94     | +1.36e-03 +/- 3.13e-03 |  4.07e-02 +/- 2.3e-03  |  3.93e-02 +/- 2.1e-03  |
 test_objective_jac_atf                  |     +2.12 +/- 3.37     | +4.02e-02 +/- 6.39e-02 |  1.94e+00 +/- 3.6e-02  |  1.90e+00 +/- 5.3e-02  |
 test_perturb_1                          |     -0.67 +/- 3.53     | -8.65e-02 +/- 4.54e-01 |  1.28e+01 +/- 3.6e-01  |  1.29e+01 +/- 2.8e-01  |
 test_perturb_2                          |     -0.49 +/- 2.72     | -8.75e-02 +/- 4.90e-01 |  1.79e+01 +/- 2.9e-01  |  1.80e+01 +/- 4.0e-01  |
 test_proximal_jac_atf                   |     -0.27 +/- 0.87     | -2.20e-02 +/- 7.14e-02 |  8.21e+00 +/- 6.2e-02  |  8.24e+00 +/- 3.5e-02  |
 test_proximal_freeb_compute             |     +0.01 +/- 0.95     | +2.27e-05 +/- 1.74e-03 |  1.83e-01 +/- 1.4e-03  |  1.83e-01 +/- 9.6e-04  |
 test_proximal_freeb_jac                 |     +1.32 +/- 0.94     | +9.91e-02 +/- 7.04e-02 |  7.63e+00 +/- 5.2e-02  |  7.53e+00 +/- 4.7e-02  |
 test_solve_fixed_iter                   |     +0.81 +/- 61.89    | +4.12e-02 +/- 3.13e+00 |  5.11e+00 +/- 2.1e+00  |  5.07e+00 +/- 2.3e+00  |

[f0uriest]

could do something like:

for coil in coils:
    errorif(not isinstance(coil, _Coil),
            TypeError,
            f"Expected object of type Coil, got {type(coil)}",
)

that way it actually says what the offending member is.