Merge pull request #29 from mark-burnett/method-refactor

Method refactor
follow law of demeter in translator
refactor: make petri generation more consistent

There used to be two general methods for creating petri nets, depending
on the context:

1. get_petri_transitions
2. attach_*

This commit replaces those with attach_transitions().  This should make
it easier to migration DAG to being a Method rather than a Task.