Add the save_scene function to output back to disk

[leouieda]

This function can be used to same a scene back into the EarthExplorer format. The output is written directly to a tar file (with or without compression). Needed to add imageio and tifffile dependencies. While we're at it, might as well use imageio instead of scikit-image for reading the tiff files. It could be useful when cropping a scene for a class or the docs. Add a warning that for general storage after a scene is loaded, using xarray.Dataset.to_netcdf is better.