Gets the el-th Legendre polynomial. If the polynomial has already been calculated
it is returned from the list. Uses the recurrence relationship to construct new polynomials
based on lower order polynomials.
public static Function getAssociatedFunction(int el,
Gets the associated Legendre function. If the function has already been calculated
it is returned from the map. Uses the Legendre polynomial recurrence relationship to construct new functions
based on derivatives of lower order polynomials.