Add Numpy-Array <-> DM-Image support

[Bejoscha]

This will maybe be a bit trickier than than the others, but supporting image-variables (at least the data content) in this simplified hybrid-concept would be great. It's possible doable by storing values as Array-tags and/or having Python/DM code create the image an passing image-ID around. The latter might have difficulties with images not being kept in scope when not being shown, though.

[miile7]

This can be done after refactoring by using a DMType then (#4). The type then decides how it is saved to the persistent tags and how it is loaded from there.

The DMImageType could then write its tag and its data separately to the persistent tags on the dm-script side. On the python side it could then load both values and create a Py_Image instance from that.