improvements to importing multiple assets

[jabooth]

import_images(), import_meshes() and import_landmark_files() have no all grown a verbose flag. Set it to True to get nice dynamic printing of the progress of asset importing (@nontas - proud? ;) ). This PR updates the load_database function to use this newly supported flag, simplifying the database loading code.

We now also raise a ValueError if the glob the user gives provides no assets. Tests are added for this behaviour.

[jalabort]

The verbose flag and the fact the we now raise a ValueError if no assets are found are two simple and great additions to the io module!!! Well done James, I really like this now ;-)