Method source will read a new "record" or value from {@link cascading.scheme.SourceCall#getInput()} and populatethe available {@link Tuple} via {@link cascading.scheme.SourceCall#getIncomingEntry()} and return {@code true}on success or {@code false} if no more values available.
It's ok to set a new Tuple instance on the {@code incomingEntry} {@link cascading.tuple.TupleEntry}, or to simply re-use the existing instance.
Note this is only time it is safe to modify a Tuple instance handed over via a method call.
This method may optionally throw a {@link cascading.tap.TapException} if it cannot process a particularinstance of data. If the payload Tuple is set on the TapException, that Tuple will be written to any applicable failure trap Tap.
@param flowProcess of type FlowProcess
@param sourceCall of SourceCall
@return returns {@code true} when a Tuple was successfully read