Add mapping function with support for multiple grid types

I'm pondering making this its own module, because it technically completes a different task.

I still need to document this task.

So for the cascadia show I built a hacked-together map for my specfic panel. it returns an array where map[row][col] = index, where index is the the # of that LED in the 4 panel with-snaking setup that I have. still pondering the API. Will keep notes here.
