Add example for grand potential phase field with strict mass conservation and improve documentation

[laagesen]

Reason

In the multi-phase grand potential phase-field model, slight variations is total system mass have been observed. These are believed to result from the fact that the evolution equation for concentration is transformed into an evolution equation for chemical potential, which introduces terms that can cause mass variation as a form of discretization error. The error tends to get worse as time steps increase in size, leading to the conclusion it is a form of time discretization error.

Design

An alternative formulation has been derived that maintains a conservation equation for concentration, and introduces a relationship between composition and chemical potential that can be derived from the grand potential functional. An input file that implements this new formulation as an example problem will be added, along with improved documentation. This input file uses existing kernels and materials to implement the new formulations, and has been tested and shown to conserve mass to within solver tolerance level.

Impact

Only adds additional example input file and documentation.