Use only one main program grt to dispatch the sub-command #54
Description
At present, C programs in PyGRT are multiple executable files, like
grt [args1] [args2] ...
grt.syn [args1] [args2] ...
grt.stress [args1] [args2] ...which is unfavorable for future development.
Inspired by Git and GMT, and given that there aren't too many programs at the moment, I should make a change as soon as possible, to make C programs as following
grt [args1] [args2] ...
grt syn [args1] [args2] ...
grt stress [args1] [args2] ...