Add method to viscosity for evaluating at a point.

This is introducing a bunch of warnings about ambiguity of types, see
http://stackoverflow.com/questions/3519282/why-is-this-ambiguity-here
Need to get rid of the old interface that's being used in
LowMachNavierStokes. Was hoping to avoid recomputing temperature -
need to think about the right API for that.
Add postprocessing computation of viscosity

Still need to register the variable before it will be output.
Register viscosity for postprocessing.

Viscosity should be plottable now.
Need to actually set the bloody value.
Don't have default value for time.

This was causing warnings in opt mode, but causing *errors* in dbg
mode. We can pass time anyway from the context, so it's not a big
deal. We can update the API later.
Need to *not* do this initialization
Add API to conductivities for postprocessing/visualization
Compute k for postprocessing

It is not enabled yet.
Register k for postprocessing

This should enabled visualization of conductivity postprocessing.
Merge pull request #176 from pbauman/mu-k-plotting

Viscosity and conductivity visualization