Specify material properties to be dumped in the Output block

[andrsd]

Like we have a list of variables that are outputted in the output block, it would be nice to have the same for material properties.

Use case:

• 3 blocks that specify thermal_conductivity
• in the output block, we'd say: show_mat_prop = 'thermal_conductivity' and the system would output it into the file of a given outputter.

Limitations: Not all formats may support dumping elemental values, that's why this will be outputter specific.

Related to RELAP-7: in their input files, people can list what material properties will go into the output. Both for visualization and/or debugging. Currently we have to set all that in the code, which is a horrible hassle. And if you want to add/remove you have to recompile(!)

[permcody]

Looks like I referenced the wrong issue in #3584. Reopening this issue.