Rename properties to use atom/expression naming

[maxitg]

The problem

Currently, the words "edge" and "vertex" are horribly overused, especially if talking about causal graphs etc., where the word "edge" could refer both to a hyperedge and the causal edge, which leads to a lot of confusion.

Possible solution

The suggestion is to use the following terms instead (which are already heavily used in the internal code):

• atom: vertex in the space hypergraph;
• expression: hyperedge in the space hypergraph;

The names for events, matches, and rules are fairly unambiguous and could keep being used as they are.

Comments

This should not break the existing API, so the old property names should still work (we have a mechanism for that, check $oldToNewPropertyNames in WolframModelEvolutionObject.m).