Use exact integer arithmetic for jacobi coeffs
Use exact integer arithmetic for jacobi coeffs

- jacobi coefficients are just integers, so we can use
exact integer arithmetic to compute them to arbitrary precision
- computing coeffs using python int with arbitrary precision
is basically the same speed as using a floating point approximation
- for L<55, the coeffs fit in 64 bit ints, so evaluation is still
fast
- using `object` dtype in polyval is slow, so for now default
to casting to int or float accordingly
- still need to improve stability of polyval, but now at least
coefficients are exact

PlasmaControl/DESC,PlasmaControl/DESC