Support serial and parallel version of HDF5

[Prof Garth Wells]

It would be helpful to make HDF5 a required dependency. This simplifies testing and allows us to move demos to XDMF.

To not by extension make MPI a required dependency, we need to re-work the HDF5 wrapper to support serial builds of HDF5.

[Prof Garth Wells]

Has this been started?

[Chris Richardson]

Now in master with tests, which will work when there is serial-hdf5 in the test system...