Add test; Make sure moments behave correctly when origin is shifted.

[PaulWAyers]

For moments, we can use as tests a function (e.g. a Gaussian) that is centered at the origin and also one centered off the origin. The change in solid-harmonic coefficients when you shift the origin of a moment expansion is analytically computable.

• There is code in Julia that shifts the origin of a solid harmonic expansion

There are also some analytic formulas available.

• see page 66
• Ruedenberg found a classical result, which is simplified in Eq. (8) of this reference
• There is also a result in this reference
• A tabulation of the coefficients for translation/rotation is available in the supplementary material of this reference

Originally posted by @PaulWAyers in #13 (comment)