Deleted abstract SimplicialComplex class.

Maybe an abstract data structure API should be added at some time in
the future.
Renamed key types.

Renamed node_key_type, edge_key_type, face_key_type and
tetrahedron_key_type to node_key, edge_key, face_key and tet_key in the
ISMesh class.
Moved get_pos functions to ISMesh API.
Added is_interface functions to ISMesh API.
Moved set_pos and set_destination functions to ISMesh API.
Added is_boundary functions to ISMesh API.
Added get_destination function to ISMesh API.
Removed locked attribute from DSC class.
Added a get_label function in ISMesh API.
Moved update functions to ISMesh API.
Removed as many calls to the get() functions from the DSC class as possible.
Simplified the collapse functions.
Added a set_label function.
Removed the dependency on the get functions completely.