Split method solveSingleCell() for easier maintenance.
Removed unused debugging output.
Work in progress fixing multidim upwinding.
Fix use of assert.
First compiling version of fixed MDU method.
Add computation of face tofs parts.

This must be done after the cell tof has been computed since they
depend on it.
Fix bugs in MDU.
Activate MDU option, making SMU method its default.
Add tracers_ensure_unity parameter.

When this boolean parameter is true (the default), tracer solutions
will be normalized so that the tracer averages will sum to one in each cell.
This behaviour is the same as before, the change is that it can now be
turned off.
Clean up dead code for MDU, update comment.
Remove unused data member.
Refactor tracer computations.

Current version executes reordered solve once for each tracer. The benefit
is a simpler code and the ability to use MDU with tracers. The cost is
potentially higher runtime, compared to doing a single sweep for all
tracers (and tof).
Fix initialization of tracer heads, and output ordering.
Merge pull request #605 from atgeirr/fix_mdu

Fixes for tof and tracer solvers