Tests normal gravity against Somigliana equation

[santisoler]

Add tests functions to compare ellipsoids' normal gravity on their surface against the Somigliana equation.

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[leouieda]

@santisoler I made a few changes here: mostly moved the function to a boule/tests/utils.py module and made some adjustments to the function (calculating sin and cos only once).

[leouieda]

We could remove the sphere test from this branch so we can merge it with tests passing. Then we can implement it in #52 so that we're confident that we can fix the sphere gravity. What do you think?

[santisoler]

We could remove the sphere test from this branch so we can merge it with tests passing. Then we can implement it in #52 so that we're confident that we can fix the sphere gravity. What do you think?

I agree. I will move the Somigliana test on the sphere to #52.

Thanks for the modifications, they are way better than what I drafted yesterday.

[leouieda]

👍🏽 feel free to merge whenever you're ready