Add in-line self attraction and loading for global tidal simulation

Self attraction and loading (SAL) forcing involves a spherical harmonic
transform (SHT) of the SSH (Ray, 1998). The spherical harmonic
coefficients are scaled by the load Love numbers, then an inverse
transformation is performed to compute the SAL field. The SAL forcing is
incorporated in the pressure gradient term along with the tidal
potential. Two different approaches are implemented for performing the
SHT:
1. SSH is gathered onto a single MPI rank and interpolated to a Gaussian
   mesh. A very fast library, SHTns, is used to compute the
   forward/inverse SHT. The SAL field is then interpolated back to the
   unstructured mesh and scattered back to the other MPI ranks. This work
   was done by @knbarton.
2. The SHT integration is performed on each subdomain and the SH
   coefficients are globally summed with an all-reduce.
Approach 1 ) performs better at low core-counts, but 2) scales better
for larger runs. This PR includes an init mode option for verification
testing of the forward/inverse SHT.

This work was done as a part of the tides task of the ICoM project.

[NML]
[BFB] for all standard E3SM tests