Allow density to change via strain-free chemistry changes

[tophmatthews]

Motivation

In some material systems, density can change due to chemistry changes as well as thermal expansion. The material property Density currently takes an initial density, given as a input param, and modifies it as a result of any displacements throughout the simulation. This limits density changes to only strain-driven (e.g., thermal expansion) thermal changes.

Design

Turn:

params.addRequiredParam<Real>("density", "Density");

into:

params.addParam<MaterialPropertyName>("density, "Material property name for strain-free density."

Really, I would like to turn this into:

params.addParam<MaterialPropertyName>("strain_free_density, "Material property name for strain-free density."

and deprecate density input parameter altogether, but would take buy in from people like @bwspenc @dschwen

Impact

More flexible Density material class.