Add S40RTS initial condition

This is a combination of 5 commits:
Adding scripts to perturb the initial temperature field according to S40RTS
Included spline.h into S40RTS_perturbation.h and S40RTS_perturbation.cc file. Added tests for S40RTS and S20RTS
Updates for prescribed S40RTS initial temperature condition
Updated cos_coeffs, sin_coeffs and spline_depths function
Updated tests