Faster exact substrate thermorefractive noise calculation

For the "exact" calculation of substrate thermorefractive noise, pygwinc currently implements quadrature integration over wavevectors following P1400084 Eq. 17. However, BV04 Eq. E7 gives the solution in terms of a special function that is implemented in scipy.special. The special function expression takes about 2 ms to evaluate, compared to 600 ms for the current quadrature integration.