Forward gravity vector to the properties object

The upscaled version of the properties contains the macro-scale gravity,
so this must be made available all the way from the wrapper and down.
Add common interface code from rel.perm. to cap.pres.

The code to find the interface from the upscaled saturation is the same
for the rel.perm. and the cap.pres. code, but these are in two different
methods, thus it has to be duplicated.

This could be refactored into something that does all the querying first
and then called some other routine, but I don't think it is worth it.
The runtime cost of querying cannot be removed without a redesign.
Capillary pressure due to hydrostatic difference
Use constant for number of phases

There are too many assumptions about two-phase (saturations should
sum to one, hard-coded number of phases returned) that the code as
it is should be able to run with more, so it might as well be a
constant.
Add contribution from fine-scale entry pressure
Derivatives of the upscaled capillary pressure
Comment on the terminology used when referring to phases
Merge pull request #21 from rolk/021_cappres

Upscaled capillary pressure (incompressible case)