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.