Method to close links to underlying dataset reader

[atedstone]

By default, there are at least two links to the underlying rasterio reader saved within a Raster object: .ds, .dataset_mask . .memfile may also count.

There are some edge cases when this is problem. The one I've identified is that Rasters with these readers still open cannot be pickled when they are being distributed to compute nodes in multiprocessing situations.

Manually calling a method to close these links once the Raster has been created is a possible solution to this problem.