fix typo
log % of transitions remaining feasible after merger
mv PropPreservingPartition.subsystems to AbstractSysDyn.ppp2pwa

to make PPP a purely geometric object, agnostic of dynamics.
This avoid .subsystems becoming a dict in case of a switched system.
attributes of AbstractSysDyn are allowed to be dicts for switched systems, but this should be contained from propagating to lower data leveles
rm subsystems arg from PPP instantiations
update part2convex, pwa_partition, other PPP uses

- part2convex now returns a tuple of a PPP and a map from new to old Regions, rather than tracking subsystems itself.
- pwa_partition returns a tuple of a PPP and a map from Regions to PwaSysDyn subsystems. It does not return an AbstractSysDyn to avoid cyclic imports wrt discretization. This also keeps it decoupled from details of abstracting dynamics.