Derived metrics may be calculated form data in raw output that is not
present in the resulting MeasurementCSV. This adds a method to provide
uniform access to raw artifacts generated by an instrument.
- Fix missing parameter in the documentation
- Clarify meaning of `sites` and `kinds` in the documentation.
- With the current implementation the `channels` argument is
useless: if `sites` and `kinds` are not also specified then all
channels are enabled anyway. Fix that by making those parameters
ignored when `channels` is provided.
