Topics on different pipelines

[Hsankesara]

          Would be helpful if we can put some useful `topics` on different pipelines so there is some semantic way of listing them e.g. data-ingestion, app-usage, questionnaire-time-features, etc

We can start adding some topics now, and add the functionality later (would be good to agree on a canonical set of core topics (can store in docs wiki or similar) but we'll need to leave it open so new topics can be added)

Originally posted by @afolarin in #67 (comment)