update init of AbstractPwa in discretize
new methods: AbstractSwitched.ppp2pwa, .ppp2sys, and attribute: ppp2modes
bugfix: U.dim can = size u + size x
return (j, subsystem) from AbstractSwitched.ppp2sys

which needs storing the PwaSysDyn (always PWA for switched systems, will be LtiSysDyn if that is passed to abstract.discretize) in the AbstractPwa, so that it can retrieve the subsystem.
This bundles all information together and ensures correct access to data, avoiding error-prone carrying around of separate AbstractPwa and PwaSysDyn objects.
It should also make life easier when exporting abstractions.
minor fixes and comments
use AbstractSwitched in discretize_switched

Simplify by keeping track only of parent Regions.
Original Regions in PWA PPPs and subsystems are returned by methods ppp2pwa and ppp2sys of AbstractSwitched.