[WIP] Add compressible EOS option to visco plastic material model

[naliboff]

An initial step towards adding the option to use the multicomponent_compressible EOS in the visco_plastic material model. In theory no functionality has changed with the initial commit and the goal is to see if all the tests pass.

@gassmoeller - two questions:

1. Following our initial conversation, does this structure look reasonable, or is there a more streamlined way to switch between the two EOS modules?
2. multicomponent_compressible currently does not support having more than one phase. Should I add this functionality to multicomponent_compressible first before proceeding with PR, or introduce logic internally within visco_plastic to handle this discrepancy?

For all pull requests:

• I have followed the instructions for indenting my code.
• I have tested my new feature locally to ensure it is correct.
• I have created a testcase for the new feature/benchmark in the tests/ directory.
• I have added a changelog entry in the doc/modules/changes directory that will inform other users of my change.

[gassmoeller]

Hi John,
Yes unfortunately there is currently no nicer way to implement this, but please make the input parameter a bit more future-proof, e.g. by naming it Equation of state and only allow the options multicomponent compressible and multicomponent incompressible for now. This way we can change the internal implementation later without having to change the input parameter.