Write multi-resolution RGBA pyramids

[forman]

Is your feature request related to a problem? Please describe.

Missing a CLI command and API functions that allow creating RGBA image pyramids from a dataset variable in the same way as it is done in the xcube server's WMTS (for the xcube viewer).

The output shall have a format that can be published by plain web servers so that frontend developers can add image layers to JS mapping clients such as Leaflet, OpenLayers, Cesium with minimum configuration effort.

Describe the solution you'd like

What:

• New CLI command xcube var2rgb
• New API function new_image_pyramid(cube, var_name, time_stamp, cmap, vmin, vmax) -> ImagePyramid
• New API function write_image_pyramid(image_pyramid, output_path) that creates/updates a directory tree {output_path}/{z}/{y}/{x}.png.

How:

• Move package xcube/webapi/im to xcube/util/im
• Move module xcube/webapi/mldataset to xcube/util/mldataset
• Make xcube.util.mldataset.MultiLevelDataset compatible with xcube.util.im.tiledimage.ImagePyramid
• Make xcube.util.mldataset.MultiLevelDataset also use xcube.util.im.tiledimage.TiledImage
• Refactor out image generation code from xcube.webapi.controllers.tiles.get_dataset_tile() and move into
  xcube.util.im.tiledimage.new_image_pyramid() that returns an ImagePyramid from given dataset, variable,
  value ranges, and color map.
• Add function xcube.util.im.tiledimage.write_image_pyramid() that traverses the tiles of an ImagePyramid
  and stores PNG tiles organised as (MapBox / GDAL style) image pryamid.
• Add CLI command xcube var2rgb allowing to generate RGB(A) imagery from given cube
  and variable. It makes use of new new_image_pyramid() and write_image_pyramid().

Additional context

Required by CfS app of CCI KE project.

[forman]

Let xcube viewer show RGB image layers