Add operationalization for "raw" vs "derivatives"

[sappelhoff]

In the BIDS derivatives workshop we discussed that "in case you are unsure whether your BIDS dataset is raw or derived, ask yourself the following question:

Was my BIDS dataset derived from another BIDS dataset?

• if yes: It's a derived dataset
• if no: It's a raw dataset

However, in most cases, people will know whether their data is raw or derived and will apply their own classification (raw or derived).

Thus, the operationalization is not a strict rule.