cpp/numeric/special functions/legendre

@1-3@ Computes the unassociated of the degree  and argument. @A@ Additional overloads are provided for all integer types, which are treated as.

Return value
If no errors occur, value of the order- unassociated Legendre polynomial of, that is , is returned.

Error handling
Errors may be reported as specified in math_errhandling.
 * If the argument is NaN, NaN is returned and domain error is not reported
 * The function is not required to be defined for $|x|>1$
 * If is greater or equal than 128, the behavior is implementation-defined