Make it easy to add a nodeset from the input file

[pbauman]

So for simple test cases for things like 3D elasticity, a convenient way to remove rigid rotations is to pin 3 different nodes in the correct ways. It's annoying that I have to separately generate this mesh and read it. It would be nice to be able to just add a node set from the input file and have it added to the mesh so we can then specify some boundary conditions on those nodes (pinned or whatever).

[pbauman]

I should add this would be easy to implement, but is in the "would be nice to have" category and not "crazy urgent/needed" since it's probably still faster just to manually generate a mesh with all the node sets I'd want for my test cases then writing this code.