general combinatorics interface

Generic interface for all combinatorial objects (everything called datastructure at the moment: domains, half-edge, IFS). All datastructures should inherit from this interface.

Please divide into sub-issues as follows:

1. create interface structure

• attributes: vertices, edges, faces
• methods: adjacent_vertices(cell), adjacent_edges(cell), adjacent_faces(cell)
• integrate interface with IFS
• for testing it should suffice that basic operations (and utils) of the corresponding datastructures is tested

2. add methods to read and write attributes from cells:

• set_vertex_attribute(self, cell, name, value)
• get_vertex_attribute(self, name, cell)
• utils that work for all datastructures should be placed in ddg.datastructures.utils:
  • implement set_valency_attr for the interface (see current version in `ddg.datastructures.halfedge.utils.set.)

3. integrate interface with half-edge

4. adapt all utils to interface (see ddg.datastructures.halfedge.utils.set, get)

• if the util is best implemented on half-edge, the util function should use the half-edge implemtation in case a half-edge surface is given

5. add general attribute mechanism #515

6. add example of creating a gauss map from faces