The `RetrievedMaps` struct captures the image and its metadata:
the last modification time and the base timestamp for the maps.
* No longer store the last modification time, called "stamp" before,
separately in the `Maps` struct
* Update methods on `Maps` to use the `RetrievedMaps` structs and
the timestamp base in particular for sampling and map marking
* Update the `MapsRefresh` implemention to use the last modification
time
* Rename some variables from `map` to `image` in the helper functions
for consistency
* Update tests and documentation
Paul van Tilburg