Implement SCFPotential.to_sympy() and possibly an instance-level to_sympy()

[adrn]

The current implementation of Potential.to_sympy() is a classmethod and therefore does not really work for potential classes whose structure depends on metaparameters, like SCFPotential. Regardless, we should implement SCFPotential.to_sympy() (even if it returns some complicated sums) as a way of validating the energy, density, and gradient functions.