Avoid recomputation of sites (and other bugfixes)

[jacobusmmsmit]

Apologies for the large, monolithic PR.

Major change: Avoid recomputation as much as possible by writing directly to the grid whenever a closest site is calculated and reading the closest site before calculation. Note, this does mean that grid cells that are not identically equal to Coord(0, 0) will be ignored during calculation for all algorithms.

This change is done by defining and exporting find_closest_site! and changing auxiliary functions to pass the grid as an argument.

Minor changes:

• Define and use Coord = SVector{2, Int} everywhere internally,
• Change the order of get_corners to allow for better looping (for upcoming edge_aux stuff),
• Reorganise tests to adhere to DRY.

Bugfixes:

• Fix auxiliary always being exact_aux in redac_voronoi.

[jacobusmmsmit]

Making more changes, hold off on the review.