Specify appropriate type for registries

[strogonoff]

Options to use as type for each bibitem:

• dataset
• registry

[ronaldtse]

As @opoudjis pointed out, ISO 690 does not distinguish a dataset or registry. Perhaps we need to update it.

[strogonoff]

I guess we should make a decision, whether to make a leap and use the “dataset” type already now or to keep IANA items using “standard” type until ISO 690 is updated?