Add, test basis_func_to_polynomial

[inducer]

This contains an ill-fated attempt to coerce the PKDO basis into pure-polynomial form for easier symbolic processing (e.g. differentiation). Ill-fated, because it's slow, doesn't work reliably (sympy.simplify), and incurs out-of-control round-off error at moderate orders. The honest thing is to just use a monomial basis for now. At some point, we might want to retry symbolic differentiation on the analytic form of the PKDO basis. This would require dropping the conditionals, doing the symbolic work, and reinserting them later.

Until then, this PR is basically here so that others don't have to make the same mistake. There's no intention to ever merge this.

cc @xywei