Implemented most of the fluid interface.

Missing is: rsMax() [postponed] and relperm() [coming soon].
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'