Use common denominations for phase positions

The rest of the simulator framework expect the phases to be in the
order listed by this enumeration, so we should be a nice citizen and
do that too.
Setup internal methods to do pres. and sat. upscaling

These functions are handled in the props object since it already has
access to the data which is necessary to do the upscaling.
Upscale pressure assuming vertical equilibrium

Integrate the pressure gradient in the vertical direction, using the
discretized framework we already have in place.
Upscale saturation by integrating vertically

We ignore variations of the area in each block; this may cause trouble
further down the line.
Merge pull request #23 from rolk/023_init

Upscale initial domain values of pressure and saturation