Add alpha dimension

[ahuang11]

Fixes #571

import rioxarray
import geoviews as gv
gv.extension('bokeh')

ds = rioxarray.open_rasterio("test_alpha.tiff")
gv.util.from_xarray(ds, nan_nodata=True)

This is the most direct way to fix it, but there is also alpha_dimension here https://github.com/holoviz/holoviews/blob/main/holoviews/element/raster.py#L620-L626, which currently does not support xarray Dataset.

[philippjfr]

Seems like there's something missing here, this only adds the test.