new: PropPreservingPartition.prop_regions

- rm arg: prop_symbols, to PropPreservingPartition.__init__
  The set of prop_symbols is now derived from prop_regions
- don't check region labels consistency if prop_regions are undefined
initialize prop_regions instead of prop_symbols
rm PropPreservingPartition.prop_symbols

because it duplicates set(PropPreservingPartition.prop_regions)