Error if non-constant density provided for incompressible flow in NS

[joshuahansel]

Motivation

For the flow Physics in Navier-Stokes, no error message is given if the user provides a non-constant functor for density, which can lead to errors due to the assumption of the formulation.

Design

I believe it is possible to check for a functor parameter if it is a constant value or not. This could be a simple error check.

Impact

Less user errors and resulting confusion.