Re-implement Obs Group Export module as data set definition in the reporting module.

There is a quick version of an Obs Group Export module, which lets you pick a construct, and it will export (as TSV) obs groups that are created by that construct.
http://modules.openmrs.org/modules/view.jsp?module=obsgroupexport

Many features are missing, as documented here: http://openmrs.org/wiki/Obs_Group_Export_Module#Limitations

Move this to the reporting framework as a data export