collate-gpads.pl should take an argument/option to set the output directory

[dougli1sqrd]

The script at utils/collage-gpads.pl is hard-coded currently to put the collated gpads in the ./legacy/ directory. This is limiting in a docker environment where we would like to specify a specific directory for all data products in the pipeline (like to a mapped directory to outside the docker image).

The script should be able to be given an output directory and the script would put the finished files there.

[kltm]

In the interim, one can just map the outside volume to that directory.