hds-v5: Incompatible changes to v5 HDF5 layout

+ Remove use of attributes indicating defined state of HDS primitive
  and number of dimensions of HDS structure arrays.

+ Move HDS hierarchy up one level in the HDF5 tree. Now the root
  HDS object corresponds to the HDF5 root group "/". Previously it
  was one level down. This extra level in the hierarchy served
  no real purpose and just made the files look "different" to normal
  HDF5 files.

All HDSv5 files should be re-created.