[firedrake] Installing firedrake with anaconda

Hi all, AFAIK firedrake-install does not work with anaconda python, because anaconda does not support virtualenv. Anaconda comes with conda environment manager instead. Perhaps an easy solution would be to optionally skip the creation of a virtualenv in the firedrake-install script. In this case, the installer assumes that the user has already created and activated an virtual environment, conda for instance, and just pip installs all the packages. - Tuomas