Update a dataitem

Dear Quincey,

You wrote:

      Deleting the link to a group should remove all the links to the datasets
within it (and therefore the datasets themselves). If the file continues to get
bigger, you probably have open IDs to the datasets, which is preventing them from
being removed from the file (until the file is closed or the application exits).

In our discussion last year I got the impression HDF5 can't really re-use the space
of deleted items within a HDF5 file (.h5)... so that if you delete datasets and
create new ones, the space taken will be old + new. Could you please explain if
there are recent changes to this or if I misunderstood in the first place...