Planetary Spherical Harmonics Community Code

First release of our planetary spherical harmonics community code.

Authors: M. Brennan (@mbrennan17), M.K. James (@mattkjames7), G. Provan (@gabbyprovan), M.F. Vogt (@marissav06) and R.J. Wilson (@rjwilson-LASP)

Thanks to: Fran Bagenal for the original idea, Masafumi Imai for sharing their original code we could test against, Stan Cowley for helpful discussions/advice and Jack Connerney for verification!