use epydoc in pc.is_adjacent docstring
don't plot PPP.adj by default
rename variable: size -> n_cells
new: __nonzero__ method for Polytope, Region

returns True if volume is non-zero
new: Region.__iter__, Region.__getitem__

depreceated: Region.list_poly
replace Region.list_poly usage with iteration and indexing of Region

outside tulip.polytope
new: polytope.is_interior
replace subtractions by a common xrange generator
handle properly case of initializing PPPwithout prop_regions
new: PPP.__iter__, __getitem__, __len__
replace PPP.regions usage with __iter__, __getitem__
plot adj instead of transitions of partitions for switched systems
new: __ne__ for Region, Polytope
replace __eq__, __ne__ None with "is None"

to avoid tests braking for objects which implement equality comparisons