Wrapper to expose extensive computations via a HERO

The herostools.actor.Processor (pls provide ideas for a name) can be used to start a computation in an isolated process via the start method. The payload itself is defined in child classes via the _run method. Since we use ProcessPoolExecutor, the _run methods are static since they only have a limited access to self.

Upon termination, the return value of _run is piped to new_data and published if it can be casted into a DatasourceReturnvalue.