Move filtering options to function arguments in get_clean_obs_function.R

[jcoliver]

To add some flexibility to get_clean_obs, default values can be added as function arguments (max.latitude, min.longitude, deduplicate, etc.).

[keatonwilson]

Min and max latitude (lonlim and latlim) added with recent @jcoliver pull request. I'll leave this open to remind myself to add a deduplicate argument.