Add load-catalog cli command

[simonwoerpel]

This is a first lay out for a load-catalog command.

It accepts an url to a catalog index json and a few options (include/exclude specific datasets), creates or updates the collections metadata and bulk loads the entities into aleph.

A few questions arise:

• should we just safely assume to map dataset names to aleph foreign_id ? This could write entities into existing collections with the same foreign_id that are originally not meant to be the dataset from the catalog
• do we want more logic (e.g. skip importing for not updated since ...)
• and something like "inspect catalog" to see some metadata without actually importing
• how to handle collections/children?