refactor: Preparatory for well to frac connection

[paveltomin]

This PR includes preparatory changes for supporting well to frac connections.

• replace CellElementSubRegion with ElementSubRegionBase where possible
• where not possible - use templates
• introduce targetRegion for perforations, to be able to connect to matrix or fracture (if targetRegion is not specified that means perforation will try to connect to all regions)
• adjust connectPerforationsToMeshElements logic - since target region is already defined, loop through subregions in searchLocalElements, also adjust visitNeighborElements and initializeLocalSearch but for now preserve old versions of those 3 functions (still used in assignUnownedElementsInReservoir), will unify later