Move shortest path function to MHN module
Use copy of find_shortest_path() to boost speed

Ditch the costly MHN import for each a-b pair
Minor changes
No longer remove itinerary coding for deleted links

Let shortest path fill in gaps on the fly during transit file generation
Minor changes
Reverting no-deletion for removed bus itin links

Presence in table generates inappropriate transit files.
Merge remote-tracking branch 'origin/master' into itinerary-fix
Revert "Reverting no-deletion for removed bus itin links"

This reverts commit 06b40ce586ab763f4bfa26513b7e20b94ad988a2.
Merge pull request #7 from nmpeterson/itinerary-fix

Missing bus itinerary segments no longer deleted from itin table