Consider exposing "nextCell" or "cellToLastChild" function

[isaacbrodsky]

nextCell or cellToLastChild could be useful for computing ranges of cell IDs, alongside cellToCenterChild

[ajfriend]

Just so I can better understand, what functionality are we hoping for? What's the use case for ranges of cell indices?

Do we want to make it easy for a user to loop through all cells (all resolutions), or just all the cells within a given resolution? Or just the cells which are children of a given cell at some resolution?

Having nextCell iterate through all cells at a given resolution makes sense to me. And if a user wanted to change resolutions, I think that should be easy enough to do with the existing parent/child functions.