introduce concept of pixel index bands

[Scartography]

• metadata gets lost when using mapchete covert to single GTiff
• use rio info --tags for verification on any tagged tile

[ungarj]

We use GeoTIFF tags to enable adding per-pixel metadata to process outputs. This works by adding a separate band where pixels with common metadata get the same index value. Furthermore a dictionary which maps between indexes and whatever metadata we want to append is passed on and written to the output GeoTIFF.

This means, however that for every process tile the index values can be different, i.e. otherwise same metadata entries can have different indexes between adjacent process tile outputs. This causes a problem when merging tiles from a TileDrectory e.g. by running mapchete convert.

By introducing a global concept of a pixel index band we can add the required logic to all the affected parts of the code:

• mapchete.io.raster.create_mosaic() should be able to merge different index bands
• mapchete.io.raster.resample_from_array() should only apply nearest interpolation to index band
• mapchete.io.raster.extract_from_array() probably should also return the index dictionary
• TileDirectory metadata should hint whether an index band exists
• process output definition shall get an index band option
• name and place of the index dictionary within the GeoTIFF files should be standardized