Attempt to clarify when a direction digit is expected and when it is not. C permits arithmetic and assignment of ints/enums so this is not able to catch many issues at compile time.

Also update copyright headers and replace 0 with H3_INVALID_INDEX when applicable.

The aperture 7 traversal tables are now file level constants, and use the same constants as elsewhere.

Also fix JK_AXES_DIGIT ordering

Also fix formatting and fix another instance where an untyped direction is used