Add a check to test if Gamma function needs to be calculated in the Polylib

At the moment is is not possible to use very high order (>100) quadrature. The reason for this seems to be the way weights for the Gauss quadrature are calculated, which makes use of a (n-1)! function, where n is the number of quadrature points. For large values of n (n-1)! is very large. This MR changes the GLL weight calculation by adding a simple test to simplify the calculation of wights.