Check that all the images in a OB have the same tags

[sergiopasra]

Currently, the tags of the OB are extracted from the first image. This sometimes cause problems when inadvertently mixing images with different tags (for example, arcs with different lamps). This is normally an error, but is difficult to catch.
numina might extract the tags of all the images in the OB and assert that are the same, aborting if they are not.