Notes on developing new commands for libgretl.

A new command should be added to both the enumeration in
lib/src/commands.h and (in the same position) in the commands array in
lib/src/cmdlist.h.

Commands can be tested out by giving them an entry in the main command
switch in cli/gretlcli.c.

The main information potentially at the disposal of command functions is:

	Z        - the data matrix
	datainfo - the data information struct