new: abs_tol attribute of ConvexPolytope, Region

to store custom absolute tolerance (experimental)
hide bbox attribute and mv Region-specific bounding box code to Region.bounding_box
copy hidden attributes in Region.__copy__
hide function reduce, mv Region-specific code to Region._reduce
update Region.is_convex to handle only Regions,

because ConvexPolytopes are by def convex.
handle only Regions in mldivide
handle only Regions in envelope and pythonize a bit iterations
replace length check by isinstance(p, ConvexPolytope)
rename Region -> Polytope
update ConvexPolytope docstring
hide cheby_ball and bounding_box functions,

use Chebyshev attributes instead
replace function is_inside with __contains__,

because is_inside doesn't exist any more