vtk output of vectors from darcy flows

[pseastham]

There is a silent error happening where vectors aren't being output correctly when Darcy's equation is being solved on 2nd-order meshes. From the obtained pressure, one can find the velocity vectors by computing the gradient.

However, something with the vtk output save is making these vectors not display at all when loaded into e.g. VisIt.

I believe the issue can be solved by looking at src/eFEM/vtkExport.jl