Naming inconsistency: attributes vs. properties

[ffl096]

Atoms in a complex can be equipped with additional user-defined data. In our code base, we sometimes call these properties and sometimes attributes. We should stick with one and rename occurrences of the other one.

• Its called properties inside the Atom ABC and in inheriting classes (Cell, Simplex, ...).
• The Complex classes call it attributes, e.g., get_simplex_attributes and get_cell_attributes.

For context: NetworkX calls these things attributes. If there is no reason against that name, I suggest going with that purely for consistency with NetworkX.