Added ELG XD selection code for the first time. Potentially requires a lot of edits. The function (and only one) added is called apply_XD_globalerror.
Added ELG XD selection code for the first time. Potentially requires a lot of edits. The function (and only one) added is called apply_XD_globalerror.

desihub/desitarget,desihub/desitarget