Add hdf5

See merge request !9

[hdf5scan] projections, scans and flat field returns now dicts with index as keys and url as value instead of lists

- add util _get_value which try to read from 'unit' or 'units' attribute the unit of the given value.

- add `Frame` and frames to filter projections, darks, flats and return projections
- start updating a few properties