hide function is_subset

restrict it to Regions
mv function is_interior to method Region.is_interior
bug fix reversed return values from is_interior
rm chebR, chebX, fulldim from ConvexPolytope.__init__ args

if anyone sets these, then it must happen within ConvexPolytope's own methods.
split function is_fulldim into 2 methods:

ConvexPolytope.is_fulldim and Region.is_fulldim.
Hide attribute full dim -> _fulldim and utilize it better to reduce computation.
Break the loop early when fulldim ConvexPolytope found in Region.
Do not compute unnecessary sum when Boolean result is desired instead.
replace is_fulldim function calls by method calls