Add basic equilibration facility

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).
Compute phase pressures in subset of cells

This commit adds support for assigning the initial phase pressure
distribution to a subset of the total grid cells.  This is needed in
order to fully support equilibration regions.  The existing region
support (template parameter 'Region' in function 'phasePressures()')
was only used/needed to define PVT property (specifically, the fluid
phase density) calculator pertaining to a particular equilibration
region.
Test cell subset phase pressure assignment.
Add reverse look-up mapping for region vectors

Class RegionMapping<> provides an easy way of extracting the cells
that belong to any identified region (e.g., as defined by EQLNUM) of
the deck.
Document public interface of phasePressures().
Document requirements of CellRange.
Include <numeric> for std::iota()

Header was missing in earlier revision.
Install crude handling of data point outside vertical span

The initial implementation of RK4IVP<>::operator() failed to take
into account the possibility that we might need to evaluate the
function outside the vertical span for which it was initially
defined.  This situation occurs, for instance, in the not uncommon
cases of the GOC being above or the WOC being below the model.

This commit installs a crude Hermitian extrapolation procedure to
handle these cases.  Refinements are likely.
Add a layer of glue to extract data from deck

This is a work in progress.
Merge remote-tracking branch 'bska/initialisation' into initialisation
Removed RK4IVP's inheritance from binary_function.

Three reasons:
 - class is a unary functor,
 - the typedefs obtained were not used,
 - binary_function is deprecated in C++11.
Add (unfinished) test case.
Created simple data for init testing.
Throw exception if datum not in oil zone.

We are not capable of handling this, and must abort.
Fix contact depths in test deck.
Still working on test_equil.cpp.
Merge branch 'master' into initialisation
Add (defaulted) gravity argument in some places.

This is done to facilitate testing, using gravity = 10 m/s^2 for example.
Modify test data.
Complete pressure test for dead-oil deck.