Fix misleading naming of path and edge types

[this-is-sofia]

Currently, we use the following functions to check the following tasks:

directed_edge_exists(v, w): checks if there is a directed edge from node v to node w or a bidirected edge between two nodes v and w
only_directed_edge_exists(v, w): checks if there is a directed edge from node v to node w
directed_path_exists(v, w): checks if a directed path from node v to node w exists, not containing any bidirected edges
path_exists(v, w): checks if a path exists between node v and node w on the underlying undirected graph, ignoring edge types

Think about a better and coherent naming. First ideas:

path_exists -> orientation_agnostic_path_exists
directed_edge_exists -> directed_from_to_or_bidirected_edge_exists
only_directed_edge_exists -> directed_edge_exists
directed_path_exists - ok.

Also add better documentation of the concept of inducing paths.