Better module doc-string for hex_utils.
Added a graph, more doc detail.

Understanding exactly how all the coordinate systems map to each other
is hard. It's even harder if you can't visualize how they all fit
together. This commit adds a graph, with labels and a translation
table, showing the relationship between the coordinate systems in an
actual hex grid.

This commit also adds greater detail into why all the systems are
useful.
Added missing _ character.
conversion function outlines.
added conversion docstrings.
Copies over code from board_types.

We'll use the code from board_types where applicable, and flesh out
the rest of the methods as needed.

I'll try to get the tests from test_board_types working before I move
on to implementing new methods, though.
Ignore todo notes
Removed references to self and cache.
Test class outline
It's spine, not spline.
fixed variable names (miss-match after copying)
Added (failing) tests.
Added hexagon_from_rso implementation to fix tests.

All the tests (that have been included so far) are now passing.
Added tests for `hexagon_from_rso`.
Add tests for `rso_from_ordinal`.
Add tests for `_find_ring`
Adds tests for `_find_spine`
Added _find_offset tests
Added _tiles_in_ring tests.
spline -> spine