Initial version

The aim is to have an easy way to find data directories in the shell, e.g. ls $(findxfel 2212 217) .

You can test it locally by using python findxfel.py, or install it with pip install .. Once it's reviewed, I'll install it in exfel_anaconda3.

Questions:

1. Run directories in raw or proc by default? Currently raw is the default, with a --proc flag to switch to proc.
2. Should it error if a run directory doesn't exist? Currently it will happily give the directory where run 9999 would be.