Add options to flow channel to output VPPs

[joshuahansel]

Motivation

For many THM applications, it is desirable to work with VPP output instead of Exodus output. Also, there is currently no way to output the numerical fluxes, which is an important output.

Design

Add two options to FlowChannel1Phase:

• create_flux_vpp: create a VPP (new type: NumericalFlux3EqnInternalValues) for the numerical fluxes
• vpp_vars: create an ElementValueSampler with the specified variables

Impact

New classes and options in FlowChannel1Phase.