Create user object to dynamically update boundaries based on changing subdomain IDs

[dschwen]

Reason

We currently update moving boundaries in the ElementSubdomainModifier user object. The logic for moving_boundary_name and complement_moving_boundary_name is painfully complicated and cannot deal properly with multiple coincident boundaries.

Design

Rip out the moving boundary logic and add a separate UO that performs the boundary updates in a second pass.

Impact

Simplified logic, improved capability.

[GiudGiud]

What's the context here? Do we have time and funding to make a category of MGs work during the simulation instead of re-creating the MG capability as UOs? I can go either way

Should the code in MGs be moved (when relevant) to utils then used by both classes?