mv import_PropPreservingPartition from spec.form to synth

to make GRSpec agnostic of details about the abstract subpackage
and because converters from systems/graphs to specs are in synth
rename import_PropPreservingPartition -> ppp2spec

rename variable self -> spec
update arguments in docstring
rename variable: disc_dynamics -> ppp

because it is a PropPreservingPartition, which is used mainly for spatial adjacency.
AbstractSysDyn is used to abstract the continuous dynamics, in particular its .ts attribute to store the transitions and labeling.