require ppp2trans argument in plot_partition

- rename nodelist argument to ppp2trans
  and if trans is a graph, then require ppp2trans to ensure a well-defined state order when converting the graph to an adjacency matrix.
- reorder plot_partition arguments
return axes handles from AbstractSysDyn.plot
bugfix: init AbstractSysDyn.ppp2ts attribute
expose is_feasible, solve_feasible, is_subset
new arg ax to LabeledStateDiGraph.plot

and plot in new axes by default, not current axes
pass ppp2ts to PPP.plot in AbstractSysDyn.plot
use is_feasible in get_transitions
check dimension consistency between matrices and domains given to LtiSysDyn
bugfix: Polytope vertex storage inconsistent with reshaped V
disable AbstractSysDyn.ts plot, add color seed argument
rewrite AbstractSysDyn

So that it stores only a single mode, to avoid attributes which may or may not be dicts.
Add more info: storing PPPs instead of (unstructured) lists of Regions.
This should help keep track of abstraction info easier, hierarchically.
Also store both the partition which preserves both PWA dynamics and propositions, as well as the original PPP, which involves only the continuous propositions.
new class: AbstractSwitched
rename AbstractSysDyn -> AbstractPwa
update AbstractPwa docstring
style pwa_partition docstring
replace indexing by enumeration
style and comments
return new2old Region map from pwa_partition

update docstring
rename var: parent -> parents
rename returned var: ppp -> new_ppp
update init of AbstractPwa in discretize
new methods: AbstractSwitched.ppp2pwa, .ppp2sys, and attribute: ppp2modes