Add tests and fix for spherical spreading centers

[danieldouglas92]

Addresses Issue #518

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.024 ± 0.004 (s=424)	1.022 ± 0.004 (s=458)	-0.4% .. -0.2%
Slab interpolation curved simple none	1.029 ± 0.006 (s=430)	1.027 ± 0.006 (s=448)	-0.4% .. -0.1%
Spherical slab interpolation simple none	1.265 ± 0.008 (s=346)	1.262 ± 0.005 (s=369)	-0.4% .. -0.1%
Slab interpolation simple curved CMS	1.099 ± 0.008 (s=399)	1.097 ± 0.005 (s=423)	-0.3% .. -0.1%
Spherical slab interpolation simple CMS	1.863 ± 0.006 (s=232)	1.859 ± 0.007 (s=254)	-0.3% .. -0.1%
Spherical fault interpolation simple none	1.273 ± 0.006 (s=347)	1.270 ± 0.005 (s=363)	-0.4% .. -0.1%
Cartesian min max surface	2.375 ± 0.016 (s=188)	2.377 ± 0.019 (s=193)	-0.2% .. +0.3%
Spherical min max surface	7.372 ± 0.026 (s=62)	7.350 ± 0.014 (s=63)	-0.5% .. -0.1%

[MFraters]

This looks good to me. Can you also add the fix for the mass conserving temperature model we discussed and update the test results?

[MFraters]

This looks nice! It would also open up the possibility in the future to allow Cartesian points to be passed to the function directly. One of the unit tests fail with this change, but I checked it and it actually gave an incorrect solution before (distance (-90,0)->(90,180) = pi, not zero). So you can update this test result.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (41ddcc5) 93.45% compared to head (774c845) 93.46%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #520   +/-   ##
=======================================
  Coverage   93.45%   93.46%           
=======================================
  Files          92       92           
  Lines        6279     6301   +22     
=======================================
+ Hits         5868     5889   +21     
- Misses        411      412    +1     

Files	Coverage Δ
...urce/world_builder/coordinate_systems/spherical.cc	100.00% <100.00%> (ø)
...eanic_plate_models/temperature/half_space_model.cc	98.16% <96.87%> (+0.31%)	⬆️
...es/oceanic_plate_models/temperature/plate_model.cc	98.27% <97.05%> (-0.71%)	⬇️

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[MFraters]

almost there, I think you forgot to add the .wb and .dat files for the test you made

[MFraters]

Great, thanks for your contribution!