Bugfix: do not compute MDU terms for noflow faces.
Implement solveMultiCell() properly.

Interface change: solver now requires a linear solver (for the multi-cell blocks only).

Implementation uses new private method assembleSingleCell(), that is a modified copy
of solveSingleCell(). Should refactor.
More refined and concise output of block info.
Make multidim upwind more forgiving of bad grids.

Now it will no longer trigger assertation failure when grids are not
edge-conformal (faulted cornerpoint grids processed by our code will
usually not be). Minor algorithm change to handle this.
Optimize multi-cell solve and add new Gauss-Seidel variant.
Removed experimental multi-cell solver code.

Since the Gauss-Seidel approach seems to be both simplest and
fastest, all parts dealing with assembling multicell systems
have been removed.
Fix typo and minimize formatting changes.
Work in progress on tracers.

 - Changed interface.
 - Read tracerheads (tracer start locations) from file in compute_tof_from_files.
 - Initialize tracerheads from wells in compute_tof.
Minor fix: do not read tracer heads unless compute_tracer is true.
Implement solveMultiCell() with Gauss-Seidel.
Tracer computations are now fixed and robust.

 - Handles noisy source terms.
 - Works with repeated solve calls (multi-cell block solves).
Update doc, only initialize tracerhead_by_cell_ if needed.
Add tracer computations (method solveTofTracer()).

Same interface as in class TofReorder.
Add call to DG tracer computations if user requests.

Used to throw with an 'unimplemented' message.
Added limiter for tracer.
Merge pull request #234 from atgeirr/tof-improvements

New and improved capabilities for time-of-flight and tracer partitioning computations.