use `world` instead of `usa` for default basemap

[stragu]

The current default value for the map argument is usa in function inat_map(), which is a very US-centric choice and creates rubbish maps in many cases when the data is not located in or close to the US.

We should use the world basemap, also provided by the maps package, and mention the other basemap available by default (france, italy, nz and usa).

This is a breaking change as it would produce different maps in existing scripts using the default argument value (but would not produce an error or warning).