Add hillslope scripts

@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](https://github.com/swensosc/Representative_Hillslopes). 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.