Made populate_gmf_agg more testable
Refactored the parallelization in the post processing phase
Fixed a bug in the case of no gmfcollections
Added a parameter nchunks to populate_gmf_agg
Fixed the default nchunks
Merge pull request #1177 from gem/populate_gmf_agg

Fixed the case of no gmfcollections and cleaned up the post processing mechanism