new: PropPreservingPartition.prop_regions
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

necozay/tulip-control,tulip-control/tulip-control,tulip-control/tulip-control,necozay/tulip-control,necozay/tulip-control,tulip-control/tulip-control,tulip-control/tulip-control,necozay/tulip-control,necozay/tulip-control