Improve treatment of coastal survey locations with custom stratifications

[AdamCSmithCWS]

Currently, there are 100s or 1000s of routes that can easily be omitted from a custom stratification, depending on the underlying land map used. Often these are routes with start locations close to the coast or on small off-shore islands.
The user can buffer the coast of their custom strata map, but this then influences the area weights applied to these coastal strata.

possible solutions:
stratify() could offer an option to include all routes within a minimum distance of a stratum.
the package could include a land-layer that we ensure contains all of the route start locations. Then users could use this base layer to modify their stratifcation to ensure coastal routes are not excluded. This same base-layer could be used to update the strata within the package (e.g., the latlong stratification that currently excludes a handful off island routes.)