User "gwdatafind" not created on Debian

Since the systemd service file explicitly requests a gwdatafind user to be existent, this must be created when first installing the package (but never removed).

While there's a %pre script in the .spec file, the equivalent is missing from the debian/ tree. I've checked some other packages's builds- e.g. condor attempts to create the user, giving the admin the option to have it pre-installed. Whether a post- or preinstall script is used doesn't seem to matter much (condor uses a post script while ganglia uses a pre one - ideas may be copied from either of them)