allow PVD/VTR output to have multiple components of displacement

[mlp6]

right now a single (default of z) component of displacement is saved, but this should be able to be augmented to be multiple for viz in Paraview

[mlp6]

And also add the ability to calculate velocity from displacement data... (commit b7efd07)

[mlp6]

All of the components of displacement (or velocity) should still be able to be added as pointData in VTK, but with a tuple of 3 floats instead of a single float to represent a vector instead of a scalar.