[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

Dev 0 2

See merge request !8

- insure that no keys are overwrite when returns the refs_urls
- close partially the nxtomomill issue nxtomomill#1