A Crunch-compatible abstract base class for Spark's DoubleFlatMapFunction. Subclasses
of this class may be used against either Crunch PCollections or Spark RDDs.
Note: Crunch can reuse a single input record object whose content
changes on each DoFn.process(Object, Emitter) method call. This
functionality is imposed by Hadoop's Reducer implementation: The framework will reuse the key and value
objects that are passed into the reduce, therefore the application should
clone the objects they want to keep a copy of.
Initialize this DoFn. This initialization will happen before the actual
DoFn.process(Object, Emitter) is triggered. Subclasses may override
this method to do appropriate initialization.
Called during the setup of the job instance this DoFn is associated
with.