allow dataflow to change operator params

[jhellerstein]

Some operators like source_interval have a parameter that we might like to change dynamically via dataflow.

[jhellerstein]

Very few operators have params other than code blocks or file names. Maybe just implement source_dynamic_interval or something?

[MingweiSamuel]

(possible to do the specific source_interval example in a roundabout way, in rust, via Streams)