Break up intake-xarray into separate drivers?

Intake-xarray lumps together several different I/O backends. Grouping drivers by what they _return_ (xarray in this case) is different from the usual pattern. We propose to break intake-xarray into intake-netcdf, intake-tiff with rasterio support (and maybe also tifffile support?), intake-png, and intake-zarr. We can maintain intake-xarray going forward as a metapackage that depends on these as a convenience and for back-compat.