Add Chunk grid to the map app

[JoshCu]

The two datasources used to produce forcings (aorc, retrospective) are stored in a zarr format that is broken up into chunks. The chunks are cubes of dimensions latitude x longitude x time, for retrospective it's something like 350kmx350kmx627hours.
Find the chunk size of the datasets, get the crs of the datasets, then plot a grid overlay on the map that displays the chunk boundaries for the datasets.