HDF5 <-> .skm/.skd converter

[johnlees]

For compatibility with pp-sketchlib a converter between formats would be helpful.

However HDF5 is a bit of a pain to replicate (could perhaps more easily be done with python) so I propose not doing this unless there is definite need.

If this does become planned, we would definitely need to wait until the .skd/.skm format is fixed, which I am not sure it is yet.