Define a property for the source of an item
I guess we like to have the information about the source of an item. For training material it seems to me relevant to limit the search to selected sources ("Show only the material that is from Programming Historian"), thus also having a facet in the MP. Currently it is implicit through the link in the attribute "accessibleAt" (@sotiris.karampatakis did you add the information on the source somewhere else?). This works not well, if we agree on having it as facet. Therefore we need to introduce a dedicated property. This can have the property-key "source". But what would be the value? Should we stick to a text value that we can directly parse to the facet, like "Programming Historian" or is it better to have a dedicated identifier? For a dedicated identifier we need a mapping, e.g. source = "100" means "Programming Historian". Opinions on this?