Enable boundary nodes to move in the `SmoothMeshGenerator`

[dschwen]

Reason

libMesh/libmesh#3399 improved the Laplace mesh smoother to permit boundary nodes to move along flat parts of the mesh boundary (and internal subdomain boundaries). This can significantly improve the mesh quality.

Current smoothing result

New smoothing result with movable boundary nodes

Design

Add a new parameter to SmoothMeshGenerator to enable the libmesh functionality. This parameter will default to the old behavior where boundary nodes are immovable.

Impact

Added capability.

[dschwen]

(ignore the aspect ratio change in the example, those are two slightly different starting meshes)