Allow datasets without distribution

[coret]

The purpose of the NDE register is to get a good insight in whats datasets are available at Dutch heritage organisations. Some datasets are not open to every one (yet), for example due to privacy concerns. The descriptions of these kind of "closed datasets" should be a part of the NDE Register.

Currently de dataset requirements require a dataset to have a distribution (datadownload). One could argue that in case of a closed datasets, a specific datadownload should be specified from which a user can understand that contact has to be made with the organisation to agree on a datadistribution.

Alternatively, we could drop the requirement for a datadownload. Looking at DCAT's scope there's a 0..* relation. Also, providing a dataset without a datadownload to Google's structured data tool is accepted (no errors/warnings).

This last option is considered preferred.

[coret]

Option considered but discarded: making the distribution's contentUrl and encodingFormat field optional (now required).

[ddeboer]

Currently de dataset requirements require a dataset to have a distribution (datadownload).

Not true. Our requirements have:

Therefore, publishers SHOULD add at least one distribution.

Which is not a MUST. I think that we should keep the SHOULD to have this as a recommendation rather than the weaker claim that distributions MAY be included.

[coret]

This sentence is ok indeed, but isn't reflected in:

• https://github.com/netwerk-digitaal-erfgoed/requirements-datasets/blob/master/index.bs#L469
• https://github.com/netwerk-digitaal-erfgoed/register/blob/main/shacl/dataset.jsonld#L54 (and possible code in register repo)