Fix pegsless inputs in beamnrc and dosxyznrc guis

Remove the duplicate element Nd (Z = 60) in the beamnrc and dosxyznrc
guis pegsless element table. Also correctly update the medium type when
reading data from a density correction file. The duplicate entry caused
incorrect assignment of elements read from a density correction file.

Improve consistency of pegsless input with that in egs_inprz and with
pegs data specification in egs_gui. Also fix a bug in the DOSXYZnrc gui
get_val procedure.

Author: blakewalters