XDM 2.7 Xyce diode parameters not up to date

[tvrusso]

Xyce 7.7 contains a number of updates to the diode model that included a new instance parameter and a number of new model parameters. The file XDM/src/python/xdm/inout/xml/schema/xyce.xml does not have those updated parameters, and therefore if I understand it correctly they will be ignored when converting from netlist formats of simulators that do have those parameters.

C.f. Xyce/Xyce#58