Write multi-resolution RGBA pyramids

[forman]

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 Cate's web API for Cate Desktop.

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. i.e. write a directory tree {var}/{t}/{z}/{y}/{x}.png.

The tool is required by CfS app of CCI KE project.