Follow-up to "Add a method to retrieve all the samples of a dataset"
Closes #149 (closed)
The all_samples methods now correctly handles the 'train' group, returning only the samples of a group if that group is given as parameter.
The 'groups' parameters contents are now verified with bob.db.base.check_parameters_for_validity.
Groups of 'legacy database interfaces' different from 'train', 'dev', or 'eval' are now handled correctly.