Add method to reconstruct SFG from a schedule

Closes #239 (closed)

Adds the method _reintroduce_delays() to Schedule, which looks through the laps for the signals in the schedule and inserts delay elements accordingly.

Also adds the method SFG.simplify_delay_element_placement() which looks through the neighbors of each delay element in order to simplify the sfg and reduce the total number of delay elements.

The Schedule.sfg property now returns an sfg reconstructed using the above functions, instead of the original sfg.