Add bing_tile_children and bing_tile_parent functions

[jagill]

Small helper functions to find the children and parent of a BingTile.

== RELEASE NOTES ==

Geospatial Changes
* Introduce ``bing_tile_children`` and ``bing_tile_parent`` functions to get parents and children of a Bing tile.

[arhimondr]

LGTM % nits

[arhimondr]

please add a message in case it ever fails, e.g.: checkArgument(newZoom <= MAX_ZOOM_LEVEL, "newZoom must be less than or equal to %s: %s", MAX_ZOOM_LEVEL, newZoom), same for other checks

[arhimondr]

Add the original exception as a cause. Propagate original message. Same for other methods

[arhimondr]

Return children ...

[arhimondr]

ditto

[arhimondr]

assertThatThrownBy(() -> { ... }).hasMessage(...);

Same for other exception checks.

You can also check the exception type and exception fields. Generally the assertThatThrownBy construct is very powerful

[arhimondr]

nit: maybe wrap, e.g.:

tiles.stream()
  .map(BingTile::toQuadKey)
  .sorted()
  .collect(Collectors.toList())

Also toImmutableList (statically imported)