Add hillslope scripts

[samsrabin]

@swensosc has various scripts related to making files/variables for use in hillslope runs. Some of these are not CTSM-specific, so they're in a separate Representative_Hillslopes repo. Some are CTSM-specific, though, so I need to add those to python/ctsm/. This will also require adding the Representative_Hillslopes repo as an optional external to CTSM.