Add unstructured grids support to `subset_data` script

It would be very useful to add the ability to create an unstructured grid (i.e. surface, domain, and/or mesh files) using `subset_data`. We are working on bringing in regional subsetting with #1892 but right now you can only subset a rectangular region (which is quite useful!).

Based on conversations with @rgknox, and not to mention the PPE team (i.e. @linniahawkins, @djk2120, @katiedagon, etc.) it would also be useful to be able to have `subset_data` take in a list of latitudes and longitudes to turn into an unstructured grid.