fix code typo in cheby_ball
make chebR, chebXc properties of Polytope, Region

- auto-computed on first access
- protected from being set outside polytope module
- no need to import cheby_ball in addition to Polytope
bugfix: enlarge matrix A if fewer rows than polytope dimension

This occurs when polytope.reduce operates before Polytope.project. If the matrix A resulting from reduction has fewer rows than the Polytope's dimension, then within polytope.projection del_dim references too large indices of norm.