Re-installation of message-ix fails - now fixed

[maximikos]

Just want to link here to issue #584 that I had when re-installing message_ix, which was a ValueError caused by a dependency. Essentially, the pint package was causing trouble, and only downgrading it to 0.18 fixed it. A conda-upgrade to 0.19.2 is not yet possible in the conda-forge channel but might soon be as described here #585.