Look for environment.yml automatically

[pont-us]

Currently, if an environment file isn't configured in the notebook or on the command line, xcetool tries to analyse and recreate the current environment. This is an awkward and unreliable process which should only ever be a last resort. Before reaching this last resort, there's one more thing xcetool could try: look for an environment.yml in the directory containing the input notebook.

I'm also tempted to suggest walking up the directory tree looking for any environment.yml in a parent directory if one can't be found in the notebook's own directory, but that probably incurs too much risk of stumbling on an entirely inappropriate environment file somewhere.