Migrate MetaDatum types and check consistency with meta_keys

Squashed commit of the following:

commit bfc6300a73288e147958cd834eb63e01b451a15a
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Dec 19 13:55:05 2014 +0100

    modified:   doc/database_and_entities/Schema_vision.graffle
    modified:   doc/database_and_entities/Schema_vision.svg

commit b3617c23ee6db4bc003f05acc28067ab15812395
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Dec 19 13:51:43 2014 +0100

    testing other too

commit 7fc574437c6b1a4f323fececdb5277c88f6a540e
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Dec 19 13:39:44 2014 +0100

    ...

commit 0c2a9c7977df2a686e577e6e0ae693f77a4285c2
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Dec 19 13:34:58 2014 +0100

    modified:   cider-ci/tasks/rspec-plain.yml

commit f4a79f81fe3bb98e75f654bcaa8a30712aaf9e01
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Dec 19 13:33:54 2014 +0100

    rubocop

commit d7000df410e3694c82929d4037c67691f5cf8d88
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Dec 19 13:33:16 2014 +0100

    Part one of consistency check

commit b17a3b2b68afb7d65b77edd5998a851270e6a7cf
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Dec 19 11:42:31 2014 +0100

    Write check for one direction, second direction and tests pending ...

commit 55128c0233a5d19bf0d4034d397de89c50f99ac7
Author: Thomas Schank <DrTom@schank.ch>
Date:   Fri Dec 19 11:00:03 2014 +0100

    Migrating meta_datum types ...