expose domains of integers equivalent to string variables in GRSpec.non_str_vars

for use during creation of the SMV file in jtlv's interface.
style
rm arg "show" from PPP.plot

following https://github.com/tulip-control/polytope:
	6f518cc5dfae4c710c688de35468f764835c6f4e
increase node_sep for pydot