avoid using PropPreservingPartition.num_regions
rm PropPreservingPartition.num_regions

- it duplicates len(PropPreservingPartition.num_regions), introducing potential for inconsistency
- is less pythonic than len(attribute)
- .regions is allowed to change (not @property or otherwise protected), which would invalidate .num_regions
- fewer attributes are easier to learn and remember
- its absence encourages using enumerate() instead, or xrange(len())
rm PropPreservingPartition.num_prop

- it duplicates len(PropPreservingPartition.list_prop_symbol) introducing unprotected dependency
- not used in a way that cannot be substituted by len()
rename: PropPreservingPartition.list_prop_symbol -> .prop_symbols

- shorter name
- "list" not included (it will become a set in future)
rename: PropPreservingPartition.list_subsys -> .subsystems

- easier to read
improve PropPreservingPartition docstring
pull proposition assignment to top of loop body
iterate over dict keys
rename: Region.list_prop -> Region.props
avoid mutable default argument
make Region.props a set, instead of list

because propositions either exist or not, no order is defined on them (i.e., labels are not labeled, e.g., in transys).
Handling lists of propositions also complicates the code.
copy Region.props where needed
make PropPreservingPartition.prop_symbols a set, instead of list
rename variable: list_prop_now -> prop_now
rename variable: dum_list_prop -> dum_prop
copy Region.props where needed in prop2partition
add more comments to prop2part
no need to pass empty list for props to Region

it has a default, which now is an empty set instead and is initialized in __init__
initialize Region.props to set of iterable argument props
update construction of Region labels in prop2part from lists to sets of props

adding a 1 at the proposition's place in the list used previously,
becomes adding the proposition symbol to the set representing the label.

adding a 0 to the list in case the proposition doesn't hold (previous),
is deleted because absence of the proposition from the set denotes this.