Populates the IBXDATA(z) array with the primary, secondary and tertiary
insurance group numbers for bill IBXIEN, if any. Fields are pulled in
this order:
GROUP NUMBER from ^IBA(355.3,Plan,2);2, where Plan is
^DGCR(399,IBXIEN,"I"_z);18. If blank, then *GROUP NUMBER from
^IBA(355.3,Plan,0);4.
MRD;IB*2.0*516.
