Added Pathways and Levels, along with various updates, small fixes and performance improvements

[peetjvv]

first of all, apologies for the huge PR

here is a couple of changes and fixes that we made in a forked version while using your package. I tried my best to keep the changes backwards compatible

most notably:

• added support for the new GTFS pathways and levels files
• added some more new extended route types that was added to the GTFS spec
• added agency_email to agency in a backwards compatible manner
• added agency_id to fare_attributes
• ported the bulk of the projects to .net standard and .net core (sorry for this big change, but it was the only way I was able to manage to get it working on my side)
• now using the System.Data.SQLite nuget package instead of the dll's (this change makes it easier to set up GTFS locally)
• updated various other nuget packages
• unfortunately had to remove Itinero.Geo (this is not compatible with .net core 2.0)
• various handy helper, factory and extension methods
• added some "bulk" sqlite database read and write methods for improved performance
• started introducing bikes_allowed for trips
• improvements to the very handy TimeOfDay value object.
• created an eventhandler on GTFSEntity (only supported by Calendar and CalendarDate atm, might expand on it in the future)
• GTFS files exported with UTF-8 encoding to support more international characters
• various code refactors

[peetjvv]

closing in favour of a pr without the azure-pipelines