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.

[landinjm]

Note that this also applies when the 0th field index is a vector field. Ideally, we should create the finite element spaces for post-processing separately. This would slightly increase memory overhead, but would fix this convoluted dependence of postprocess.cc and equations.cc. It wouldn't change the fact that post-processed fields are only calculated during outputs.