vtkHDF: transient support

This MR introduces support for reading transient ImageData and UnstructuredGrid data in the VTKHDFformat. Given the extent of the changes, the VTKHDF major version number has been incremented to 2.

Schematically, the extra Steps group that contains metadeta dictating how to read the transient data looks like this:

Particularities:

• Transient vtkImageData have an additional time dimension added to the arrays which should have the following shape [~nSteps, ZDim, YDim, XDim, NComponents] (where the nSteps dimension does not necessarily have to have as many entries as time steps and can use the offsetting mechanism to replay data without rewriting it)
• Transient vtkUnstructuredGrid have flattened time-space arrays that are read in using the offsetting mechanism
• The Steps/Values array holds the time values for each one of the steps
• AMR type structures do not yet support transient data in the format

Specific documentation related to the evolution of the format will be included at https://kitware.github.io/vtk-examples/site/VTKFileFormats/ once this MR has been merged.

Some gifs showing the new behavior in ParaView: