Make MrsVariable sortable.
Make EPs and Nodes sortable, remove sorted_eps()

Similar to MrsVariables, EPs and Nodes are now sortable, based on:
 1. cfrom
 2. cto
 3. -is_quantifier() (meaning quantifiers come before quantifiees)
 4. pred.lemma (just as a fallback)

Now that we can just do sorted(eps), the sorted_eps() function is removed from pyDelphin