Potentially improve the interface to GegenbauerPolynomial

[dvandyk]

Adapt GegenbauerPolynomial:

• check if the exponent alpha can be made constexpr
• check if we can template the class with respect to the order parameter.
• adjust all usages of GegenbauerPolynomial in the code.

[dvandyk]

• check if the exponent alpha can be made constexpr
• check if we can template the class with respect to the order parameter.

Both of these should be possible, since as off C++20 non-type template parameter types can be of floating point type!