Add different meshing targets for each group in ConformalMesher

[lmdiazangulo]

Different groups, which represent different objects can have different requirements. Some possible targets might be the following

• StructuredVolume. This would be the target for dielectrics.
• StructuredSurface. For SGBCs until Conformal SGBCs are available.
• StructuredLines. For current implementation of MTLN. Force structuring on the cells that they cross.
• ConformalVolume. Defining PEC Volumes. Having less restrictions than conformal surfaces (see Implement a less restrictive snapping for conformal volumes. #30).
• ConformalSurface. For thin PECs and future Conformal SGBCs.
• ConformalLines. For slanted wires.