Check edge types allowed given a graph_type passed in export functions

[adam2392]

          What happens if a user has `graph_type="dag"` instead here? Will this function detect that there are `o->` edges and prevent that? Clearly the imported file does not represent a DAG so we shouldn't let the user make this mistake. Or we should have clear tests to show what happens when you import a graph under a different graph assumption than the one it was created with

Originally posted by @jaron-lee in #60 (comment)

[adam2392]

This is mainly for robustness and error-checking within the export submodule