Remove source_Sv_path from add_splitbeam_angle

[leewujung]

Right now there is a source_Sv_path variable created within add_splitbeam_angle, which is then used in the add_angle_to_ds function to add the newly computed angle variables to an existing dataset. Wouldn't the way to add new variables be the same for either an xr.Dataset object in memory or lazy-loaded from a path?

This was originally from #1216 (review)