ntw class structure
===================

This commit features the introduction of the ntw class along with utility functions and features for obtaining data from Vienna's Wiener Linien public transit network.

* The package now contains a snapshot of Vienna Wiener Linien network data (and development code for producings that snapshot).

* The newly introduced ntw class summarizes public transport networks as node and edge definitions.

* A number of utility function (coercion, testing, summarizing, plotting) are provided for ntw objects.