Properly map internal derived from/contributes to.
Call parent get_parameters method.
Refactored append method.
Replaced deprecated safe=True.
Merge github.com:wlanslovenija/datastream
Fixed test typos.
Fixed tag mapping methods.
Merge github.com:wlanslovenija/datastream
Don't round timestamps and don't mind gaps.
Implemented derive_from descriptors.
Merge github.com:wlanslovenija/datastream
Added test for chained derived streams.
Handle source streams with lower granularities.
Fixed indentation.
Fixed comments.
Added counter_reset operator.
Added TODO.
Added counter_derivative operator.
Enforce stream order for counter_derivative.

This will be required when we implement backward processing of
the derived stream to process the input streams in proper order.
Fixed typo.