In external concepts table: change column name "label" to "external_id"

[rue-a]

Links to the external concepts are built by concatenating the uri_prefix from the concept's source with the concept's ID. E.g., concepts from wikidata have the prefix https://www.wikidata.org/wiki/ and an ID in the style of Q<###> (e.g., Q65522654). In the LUCKINet ontology, these (external) IDs are stored in the labels-column, which does not really makes sense, since they are no labels.

As a result, I'd recommend to either rename the column to "external_id" or to add a column named "external_id".

I think the same problem occurs in external classes.

[rue-a]

Thinking about this, this is a chance to reduce some complexity. Instead of adding a column "external_id", which then has to be combined with the according uri_prefix, we could simply add a column "url" or "iri" that holds the URL (or IRI) of the according external concept.

I think that would make the process of adding an external concept less error-prone, since one then does not have to disentangle the URL into prefix and external_id, which - in the end - get concatenated again. When simply copy-pasting the external concept's URL it is hard to miss a symbol or something.

Also that would possibly make the uri_prefix column we added because of Issue Enforce value for homepage in sources obsolete, therefore making the package easier to use (understand).