`postprocess.cc` needs at least one explicit time-dependent variable to work

[landinjm]

When trying to include post-processed fields for time independent variables you run into a divide-by-zero error. This likely has to do with the initialization (or lack thereof) of the invM used to calculated post-processed fields.

Including an auxiliary variable does not fix this issue. Including an explicit variable does.

[landinjm]

This is due to the post-processed fields piggy-backing on the main set of fields to use their DOFhandlers.