Clarify chapter on dataset members in a dataset series

[matthiaspalmer]

We think that the current form of chapter 7.8 is problematic, for instance:

1. It gives the impression that it introduces a new class as it states it is a subclass of dcat:Dataset, this is not the case
2. It should not express frequency (dcterms:accrualPeriodicity)
3. Quite a few suitable properties are missing, e.g. dcterms:spatial and dcterms:temporal

We suggest rewriting this chapter and talk about which properties from dcat:Dataset that are expected and which are discouraged.