High memory usage

When running on MC with several systematics turned on (not nearly all of them though, I haven's split the JES sources), memory usage goes up to nearly 7 GB. That might be a problem for running on some clusters...

Note that I did try to follow the recommendations at https://cp3.irmp.ucl.ac.be/~pdavid/bamboo/userguide.html#ordering-selections-and-plots-efficiently