Speedup NN body wave calculation by using special functions

• gravg_pwave and gravg_swave use Bessel and Struve functions to speed up the calculations. nan is returned at high frequencies where these functions have numerical errors. For the Cosmic Explorer interferometers this is above about 300 Hz where NN is negligible.
• The original numerical integrals have been kept as an exact option.
• Speeds up the CE Newtonian body wave calculations by a little more than a factor of 15 and the total CE calculation by a little more than a factor of 5. The other interferometers do not currently use these functions.

assigned to @evan.hall

approved this merge request

added 19 commits

• 5be2babd...c1d60e95 - 18 commits from branch gwinc:master
• cabd299d - speedup NN body wave calculation by using special functions

Compare with previous version

approved this merge request

merged

mentioned in commit 6a907b19