added mongodb map aggregate writer to write to mongodb
assigned converter instance to new partitions in define partitions
made Metric and Operation serializable
add working test etl app
enhanced mongodb aggregate writer to compute and update metrics for existing tuples
Merge branch 'ETL' of https://github.com/DataTorrent/Malhar into ETL
Merge branch 'ETL' of https://github.com/DataTorrent/Malhar into ETL
changed to mongodb map aggregate writer