Get all the names of the available datasets in an `Engine` or `H5CUDS`

[itziakos]

It would be useful to be able to get that information from the Engine or H5CUDS in order to create GUI dialogs for the user to select which data sets needs to be loaded in tools like Mayavi2.

something like get_dataset_names() that returns the list of the available dataset names.

[roigcarlo]

This is fixed in #206