add a test case for gwb_grid in sphere.

[alarshi]

This PR adds a test case for using sphere in gwb_grid (#526)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ffbe304) 93.12% compared to head (bba5f2b) 93.11%.
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #616      +/-   ##
==========================================
- Coverage   93.12%   93.11%   -0.01%     
==========================================
  Files          92       92              
  Lines        6324     6332       +8     
==========================================
+ Hits         5889     5896       +7     
- Misses        435      436       +1     

see 7 files with indirect coverage changes

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[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.017 ± 0.007 (s=448)	1.017 ± 0.004 (s=439)	-0.2% .. +0.0%
Slab interpolation curved simple none	1.018 ± 0.005 (s=496)	1.015 ± 0.006 (s=392)	-0.4% .. -0.1%
Spherical slab interpolation simple none	1.167 ± 0.013 (s=381)	1.166 ± 0.005 (s=393)	-0.2% .. +0.2%
Slab interpolation simple curved CMS	1.057 ± 0.005 (s=443)	1.056 ± 0.004 (s=411)	-0.1% .. +0.1%
Spherical slab interpolation simple CMS	1.545 ± 0.008 (s=274)	1.545 ± 0.010 (s=311)	-0.1% .. +0.2%
Spherical fault interpolation simple none	1.171 ± 0.006 (s=378)	1.171 ± 0.005 (s=393)	-0.1% .. +0.1%
Cartesian min max surface	2.316 ± 0.024 (s=178)	2.313 ± 0.017 (s=213)	-0.4% .. +0.2%
Spherical min max surface	7.272 ± 0.054 (s=61)	7.294 ± 0.053 (s=65)	-0.1% .. +0.7%