Simplify how batch jobs find Python tools
The idea here is that all the Python code which runs in a calibration job (like nbconvert, ipcluster & Sphinx) should be the versions in the same Python environment used to launch xfel-calibrate. This is achieved by launching tools with path/to/python -m module instead of scripts on $PATH. This is a common pattern we use to run Python modules in a specified Python installation.
The motivation is to have better control over what's going on - we don't want to silently run another version of one of these tools which happens to be found on PATH. This also makes it easier to install pycalibration in an environment (#26).
Closes #26 .