bugfix: separate si \cap s0 during discretization
bugfix: separate si \cap s0 during discretization

to avoid adding disconnected Regions to the partition.
Introduced indirectly in:
  5b1e9681918739b276a221fcc1fd6eebfd058ce3
and:
  e2d631634cc7eac838fc06f0f219d9444d17e847
because taking the union of multi-step presets can result in a disconnected Region s0, so the intersection with the desired initial set si will be disconnected (that intersection is isect).

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