Be able to convert a batch to a stream explicitly

While most nodes can handle batch or stream data there are a few cases where an explicit conversion is appropriate.
1. For a UDF that can only handle a single edge type.
2. For streaming aggregate functions, i.e. movingAverage and cumulativeSum both reset counters on batch boundaries. A user may wish to remove those boundaries explicitly.

No files, tests, or entry points are identified in the issue. Start by locating Kapacitor's batch and stream data-handling paths and determine where an explicit conversion could be exposed; done means both listed use cases can request conversion and the behavior is covered by tests.

Written by the indexing model from the issue text.