make isAbout mandatory for datasets

[surchs]

For datasets, the isAbout field seems to be the best place to store (complex) context level annotations:

Different entities (biological entity, taxonomic information, disease, molecular entity, anatomical part, treatment) associated with this dataset

At the moment isAbout is not a required field in the DATS dataset schema. We have discussed to make it a required field within CONP.

This could happen either by

• adding isAbout to the "required" keyword of the schema
• adding the isAbout requirement to our custom DATS validator

We should discuss which of these to go with.