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
There is a merge conflict here. Please just remove the conflict (or just replace this file by the "true" output file) and push this as an additional patch to your branch so it shows up on the merge request (no need to re-squash the patch if this is the only thing).