Derivation Redefined

[cbode]

I propose narrowing the definition of 'derived' to clarify its purpose.

Derived: "a datastream that is calculated from an existing datastream." This is critically important, because it means you can go back and recalculate the datastream in the future. It doesn't matter if the calculate is simple or complex or whether the calculation occurs on the logger or server-side. It only matters that the source data is recorded as a datastream.

Datastream schema has 2 fields for derived. Remove "derived" boolean, keep "derivation_description". Add new field, "derived_from_datastream_ids": [array of object ids].

Note: a derived datastream should not have any "thing_id" objects. They should be mutually exclusive. However, we are not enforcing that at this time.