Updated excitation moment files to new PlanetMag convention

This release updates the file format for excitation moment files read in with read_Benm, as well as the function call signature. The excitation moments file now contains a descriptive name for each excitation, and an ndarray of these names is now returned by read_Benm.

This update breaks a key piece of existing infrastructure. All existing implementations of MoonMag will need to be updated to function again, but need only update the function signature with an extra output.