Adds support for the A5 dggs

[alpha-beta-soup]

Closes #68 by adding support for the A5 DGGS.

A5's polygon_to_cells operates on a single ring, so the function is called multiple times in the presence of holes; the cells representing each hole are then subtracted from the primary set.

The same function also returns a compacted set. Although it would be useful to retain the compacted set and avoid uncompact, the issue in this case is that the user-defined parent resolution -pr sets an upper bound on the compaction, but the function does not have a limit.

Following the existing codebase, hex strings are used throughout. This should be avoided in the future, and a user should be able to specify the desired output type.

[alpha-beta-soup]

Sample input:

--geo polygon, shown with parent cell IDs in colour:

--geo polygon --compact, shown with parent cells IDs in colour:

--geo point --compact --keep_attributes, shown with a land cover class in colour:

--geo polygon --compact --keep_attributes, shown with a land cover class in colour: