Included grid generator scripts

This MR includes 2 scripts I made to either get all the quadkeys within a bounding box with Mercantile or to fill an irregular polygon (countries in GADM) with the polyfill feature from Babelgrid adapted to do some parallelization and consume less memory. Both output a list of quadkeys as a text file or csv if preferred.

I am aware that these 2 scripts may look a bit long for a single MR but it would be great to have some input anyway. Hopefully this can help someone else to produce grids until we come up with a better solution.

\approve @cnievas @ds @fd