renamed output download filename to IMOS-aggregation-<creation date/time>.nc
Fixed test in config.groovy
Changes based on review

I wrote the changes in collaboration with Angus to demonstrate what I
meant and in order to keep the tests passing because of the JSON
serialiser, yeah that's right with an s.
Changes based on review

Extract output filename formatting from within Worker.getCmd and replace
comment with method with regard to filename input via command line.
Merge pull request #44 from aodn/change_output_filename

Change output filename