CellUnion broken, because of missing implementation of get_cap_bound

[calestyo]

All regions should have a get_cap_bound method.

But it seems to be only implemented for Cap, Cell and LatLngRect... while it's missing for CellUnion.

This makes CellUnion effectively broken, as it cannot be used with most other things like RegionCoverer.

[calestyo]

I'd guess the code could be simply taken from the C++ version:
https://github.com/google/s2geometry/blob/master/src/s2/s2cell_union.cc