Add mule dependency to payu/dev environment

[jo-basevi]

Mule is required as one of the dependencies for running um2nc-standalone (see #34).

It is also required as one of the dependencies for update_landuse.py scripts when running ACCESS-ESM1.5 configs with payu (see https://github.com/ACCESS-NRI/access-esm1.5-configs/blob/dev-historical%2Bconcentrations/scripts/update_landuse.py)

[jo-basevi]

The update_landsurface script also had an import for xarray which also is not in the environment, so I've added it as a dependency

[CodeGat]

Looks good. Never knew about the mule @ git+https... syntax, which is cool. However, is the #subdirectory=mule bit a comment or something special?

[jo-basevi]

The mule is a subdirectory of this repository: https://github.com/metomi/mule, and I think #subdirectory will only pip install the sub-directory rather than the whole repository