adding an option to upgrade all the python packages in virtualenv

[francispoulin]

Following a discussion from a couple of weeks ago, David suggested the following:

"We should have a firedrake-update --environment to also upgrade all the python packages in the virtualenv."

[wence-]

FWIW, I think that although this would be nice to have, there is currently no way we can do it safely. The problems are that pip which we use to install the python packages doesn't have a dependency resolver right now. Furthermore, there's no guarantee that upgrading everything won't break things. See the discussion in pypa/pip#59, pypa/pip#988, pypa/pip#3819.