Initial version of segmented well model.

This computes pressure differences with respect to the bottom-hole pressure
for each well perforation, based on the well flows. It is explicit and not
implicit, using the previous time step's flow rates to calculate the necessary
densities.
Added a test program for SegmentedWellModel.

The only currently tested case is a simple water injector.
Documented interface for SegmentedWellModel.
Fix mixture calculation for volatile oil case.
Modified test case to match MRST test case.
Bugfixes in treatment of perforation depths and final accumulation.
Merge branch 'master' into seg-well-model
Renamed SegmentedWellModel->WellDensitySegmented.
Proper memorymanagement in test program.
Fix indexing error in if-test.
Use correct number of items in initialiser.
Merge pull request #78 from atgeirr/seg-well-model

Segmented well model