Remove an instance of EOL whitespace.
Implement static gravity potential in DerivedGeology.

This is a stepping stone towards implementing gravity support in
ImpesTPFAAD.
Construct (and store) gravity operator on grid.

The gravity flux of phase 'i' across an (internal) interface is then
given by

    gflux = mob_i * (G * rho_i)

assuming 'rho_i' contains the phase density (at reservoir conditions)
of phase 'i' and 'mob_i' is the phase mobility of phase 'i' at the
interface.
Add tentative implementation of gravity flux.
Merge remote-tracking branch 'atgeirr/master'
Add first, working gravity solution

When run as

    ./test_impestfpa_ad permeability=1.01325e15 ref_pressure=1.0e-5 \
    mu1=1000 mu2=1000 rho1=1 rho2=1

this example reproduces the expected solution (a one-Pascal pressure
increase per cell).

Note that this is an incompressible case, so 'test_impestpfa_ad' must
be compiled with the pre-processor symbol

    HACK_INCOMPRESSIBLE_GRAVITY

defined to a true value (e.g., -DHACK_INCOMPRESSIBLE_GRAVITY=1)
Enforce row-major ordering of the pressure Jacobian

Otherwise, we will use the transposed Jacobian as a coefficient
matrix in the 'linsolver_'.  This is wrong when solving compressible
problems for which the Jacobian matrix is not symmetric.
Merge remote-tracking branch 'bska/master'
Start fleshing out overall structure of Newton loop

Specifically,
  - Extract method solveJacobianSystem() for solving a single,
    system of linear equations (i.e., the Newton step).
  - Extract method residualNorm() for computing the residual norm.
    Used in loop control.
  - Stub out a method for computing fluxes.

Compile and run-tested on a linear, gravity-driven flow problem.
Merge remote-tracking branch 'atgeirr/master'
Initial untested implementation of relperm() methods.
Merge remote-tracking branch 'bska/master'