Implemented support for derived streams.

Derived streams are automatically populated by data from other
streams, postprocessed by an operator. They can currently be
used to compute a sum of multiple different streams or a derivative
of another stream.
Require operators to implement update method.
Made 'derived_from' and 'contributes_to' tags optional.
Removed empty method.
Changed the order of timestamp and value.
Changed the way user-provided parameters are validated.
Changed operator parameter passing.
Improved readability.
Fixed expression and inc should be set.
Merge github.com:wlanslovenija/datastream