Allow renaming of the aggregate output fields

[eranra]

Allow flexibility to aggregate output fields. For example today the name "aggregate" is hardcoded, we want to allow changing the name of the field

[eranra]

@KalmanMeth can this be closed??

if I understand correctly with the new customizable pipline capabilities we can add "transform generic" post and modify field names?

[eranra]

@KalmanMeth maybe there is a missing feature to get all fields and modify only subset --- today we need to specify all fields that we want to generate as output and we do not have "copy all input fields to output fields" ... right>?? this might be needed

[KalmanMeth]

This feature is now covered by the transform_generic module. PR #156