Add support for NeTEx public transport

[EmilioOldenziel]

Currently the GTFS format is the only possibility of importing PT definitions, routes and timetables.
NeTEx is the official european standard to exchange public transport information, most eurpean pt authorities are also officially adopting this standard.
It defines the network, routes, lines and timetable in a single xml file.
NeTEx has a general european definition including xsd files: https://github.com/NeTEx-CEN/NeTEx

And also country or region specific dialects of NeTex exist:
The Netherlands: https://github.com/BISONNL/NeTEx-NL
Nordics: https://github.com/entur/profile-examples/tree/master

My suggestion would be a conversion script for netex2pt (which are quite close in format) with file input --netex and with an optional dialect flag --netex-dialect [NL|NORD|FR]which can deal with the dialect specific (XSD) parsing.

[namdre]

As discussed, feel free to send a pull request or post questions if something is unclear about the implementation.