Segmented well model

This adds a class for an explicit, segmented well model. The task of the well model is to compute the pressure differentials between the bottom-hole pressure and the perforations. In this class it is done based on explicit flow rates, that are used to compute the wellbore mixture.

The class is not yet used, but is intended for the fully implicit black-oil simulator.

A simple unit test is included.

Please hold off merging this, as I'd like to add some more unit tests. Feedback is welcome.