Develop the capacity for a user-defined stratification

[AdamCSmithCWS]

I've heard from a few users that it would be useful if the package would allow a user to input their own custom stratification layer. For example, load a spatial object, such as a hexagonal grid, and have the package stratify the BBS data based on this spatial object.
The package would need to group the BBS data, estimate the area-weights for the custom strata, and then integrate the spatial stratification object into the trend mapping function.