The class VTKFile should be renamed to PVDFile

[jonnybolton16]

When calling firedrake.output.VTKFile(), the file that is created is actually a .pvd file.

.vtu files aren't created until the VTKFile.write() method is called.

.vtk files are never created.

I'll concede that ParaView is based on VTK, so there is certainly a link back to VTK which can be documented, however I would argue that the class should be renamed to PVDFile, to make it clear that this function will create a .pvd file, and not a .vtk file.

[connorjward]

VTK is the underlying technology used to write the file. It is even embedded into the metadata that we write. Therefore I would argue that calling the object VTKFile is at least not wrong. Also the docstring makes it clear that a .pvd file is produced.