Use gunzip instead of zcat in pull_files.py

Now that the xrdcp option is available in pull_files.py, we could also make the script use gunzip and mv instead of zcat and rename so that HiggsDNA also works out of the box on a local MacOS machine. zcat behaves differently on MacOS and rename doesn't exist so the user would have to do those manually.

Edit: I take back the part about rename. It can installed from brew: https://formulae.brew.sh/formula/util-linux