Set up new DAMNIT folders with current exfel-python environment

[takluyver]

At present, we hardcode the Python environment to use when setting up a new DAMNIT folder. This has to be updated manually, as in #315, which is easy to do 👍 but also easy to overlook 👎 .

So we should automate this, either by looking up what the default exfel-python module points to, or by looking for the 'current environment' symlink which @RobertRosca might create.