referential integrity tests
add referential integrity error class
change entity factories to loaders (load one line at a time into maps)
check range on all numeric fields
gtfs loader library changes

Deduplicate strings
Improve missing column errors
Pull out required field check into shared method
Check time ranges
Add comments
Date parsing
add method to get URL fields
replace id indirection with object pointers.

also added Fare and Service classes for joins (not used yet)
load service calendars and fares as joined tables
stub, comment