Add underground Rayleigh NN calculation to newtonian.gravg_rayleigh
This follows the equations in Amann et al. (2020) RSI 91(9) 094504. The functionality for above-ground detectors should not be affected.
This follows the equations in Amann et al. (2020) RSI 91(9) 094504. The functionality for above-ground detectors should not be affected.