xml.etree.ElementTree usage

[gdmcbain]

This is for #620, being the first part of the splitting up of #610.

[gdmcbain]

With this, the example from #620 gives:

{'Reference': 'XML'} 
                   /Xdmf/Domain/DataItem[@Name="Point Data"]
               {'Reference': 'XML'} 
                   /Xdmf/Domain/DataItem[@Name="Point Data"]
               
[[[0. 0. 0.]
  [1. 0. 0.]
  [1. 1. 0.]
  [0. 1. 0.]]

 [[0. 0. 2.]
  [1. 0. 2.]
  [1. 1. 2.]
  [0. 1. 2.]]]

[[[0. 0. 0.]
  [1. 0. 0.]
  [1. 1. 0.]
  [0. 1. 0.]]

 [[0. 0. 2.]
  [1. 0. 2.]
  [1. 1. 2.]
  [0. 1. 2.]]]

which is correct.

It's still not possible to meshio.read('ref.xdmf') because meshio mistakes the DataItem Name="Point Data" for a Grid; handling that requires the full #610 of which this is the first part.