DL LM notebook and location of data

[KrisThielemans]

01* currently says

if not (
    Path("..")
    / ".."
    / "data"
    / "PET"
    / "mMR"
    / "NEMA_IQ"
    / "20170809_NEMA_60min_UCL.l.hdr"
).exists():
    retval = subprocess.call("../../scripts/download_PET_data.sh", shell=True)

Best to use download_data.sh -p, and use the examples_dir function as below. Note that re-executing that download should be fairly painless, so not sure if you need the if, @gschramm