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This commit adds a simple facility for calculating initial phase
pressures assuming stationary conditions, a known reference pressure
in the oil zone as well as the depth and capillary pressures at the
water-oil and gas-oil contacts.
Function 'Opm::equil::phasePressures()' uses a simple ODE/IVP-based
approach, solved using the traditional RK4 method with constant step
sizes, to derive the required pressure values. Specifically, we
solve the ODE
dp/dz = rho(z,p) * g
with 'z' represening depth, 'p' being a phase pressure and 'rho' the
associate phase density. Finally, 'g' is the acceleration of
gravity. We assume that we can calculate phase densities, e.g.,
from table look-up. This assumption holds in the case of an ECLIPSE
input deck.
Using RK4 with constant step sizes is a limitation of this
implementation. This, basically, assumes that the phase densities
varies only smoothly with depth and pressure (at reservoir
conditions).