Add functionality for importing precomputed moments

[williamjsdavis]

As the title says, it would be good to have support for importing precomputed moments for later use in the inversion process. This functionality would require designing some class or function that interfaces with MomentClass.

[williamjsdavis]

This also intersects with issue #2, as class attributes that are usually defined in ObservationClass would be undefined.

[williamjsdavis]

Also need to add a check on

secn-PR/src/MomentClass.m

Line 27 in bb14911

evalPoints

to enforce row vs. column vector. Or enforce it later in the inversion process