PROCEDURE: add_bdir,dat,source PURPOSE: Adds magnetic field direction [theta,phi] to a 3d structure The new structure element will be a two element vector [theta,phi] with the tag name 'bdir'. INPUT: dat: 3D data structure (i.e. from 'GET_EL') [source] : String index that points to magnetic field data. Notes: Magnetic field data must be loaded first. See 'GET_MFI'