URL stability of reference data

[VladimirAlexiev]

(continuation of #61 and statnett/Nordic44#52).

https://energy.referencedata.eu/PropertyReference/Equipment.inService uses an obsolete namespace:
nc: <http://entsoe.eu/ns/nc#>
According to https://github.com/Sveino/Inst4CIM-KG/blob/develop/rdfs-improved/CGMES-NC/ttl/RemedialAction-AP-Voc-RDFS2020.ttl, the latest namespace is this (and it will never be changed):
nc: <https://cim4.eu/ns/nc#>

If that is the only case in reference data then it's not a big deal.
But I suspect it is not.

Reference data should use only the latest, un-versioned semantic URLs for all of:

• instances (this is what reference data is all about)
• classes
• properties