Change to CoilSetLinkingNumber

[singh-jaydeep]

The current linking number objective returns, for each coil, the sum of Gauss integrals with respect to every coil. This includes a coil's integral with itself, which can be nonzero even if there is no linking. One of the changed tests has such a coilset. This PR removes those terms from the objective.

The stage 2 notebook uses this objective, so I can rerun it and see if the results change.

[github-actions]

Memory benchmark result

|               Test Name                |      %Δ      |    Master (MB)     |      PR (MB)       |    Δ (MB)    |    Time PR (s)     |  Time Master (s)   |
| -------------------------------------- | ------------ | ------------------ | ------------------ | ------------ | ------------------ | ------------------ |
  test_objective_jac_w7x                 |   -0.26 %    |     4.251e+03      |     4.239e+03      |    -11.05    |       30.68        |       28.10        |
  test_proximal_jac_w7x_with_eq_update   |    0.48 %    |     6.808e+03      |     6.840e+03      |    32.43     |       151.23       |       151.01       |
  test_proximal_freeb_jac                |    0.02 %    |     1.353e+04      |     1.354e+04      |     2.34     |       81.13        |       80.29        |
  test_proximal_freeb_jac_blocked        |   -0.22 %    |     7.895e+03      |     7.878e+03      |    -17.03    |       68.41        |       68.85        |
  test_proximal_freeb_jac_batched        |    0.08 %    |     7.888e+03      |     7.894e+03      |     5.94     |       68.37        |       68.32        |
  test_proximal_jac_ripple               |   -1.23 %    |     3.777e+03      |     3.730e+03      |    -46.55    |       53.49        |       53.89        |
  test_proximal_jac_ripple_bounce1d      |   -1.71 %    |     4.060e+03      |     3.990e+03      |    -69.32    |       67.16        |       67.63        |
  test_eq_solve                          |    0.16 %    |     1.829e+03      |     1.832e+03      |     2.91     |       51.99        |       51.88        |
  test_objective_quadratic_flux_jac      |    0.55 %    |     1.877e+03      |     1.887e+03      |    10.32     |       34.13        |       34.33        |

For the memory plots, go to the summary of Memory Benchmarks workflow and download the artifact.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.35%. Comparing base (187e98e) to head (97bac42).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2308   +/-   ##
=======================================
  Coverage   94.34%   94.35%           
=======================================
  Files         101      101           
  Lines       29042    29043    +1     
=======================================
+ Hits        27400    27403    +3     
+ Misses       1642     1640    -2     

Files with missing lines	Coverage Δ
desc/coils.py	96.84% <ø> (ø)
desc/objectives/_coils.py	99.34% <100.00%> (+<0.01%)	⬆️

... and 1 file with indirect coverage changes

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