Support for multi-band rasters with nulls in some (but not all) bands

[alpha-beta-soup]

Situtation: 4 band raster where there may be null values in some bands that are not null for the equivalent pixel/s in the other bands.

Outcome: error reading the intermediate data in step two https://github.com/manaakiwhenua/raster2dggs/blob/master/raster2dggs/h3.py#L266

It may be that we should write this intermediate data differently, or else note this limitation.

Work-around: split such rasters into multiple files and index independently.

Another "solution" would be to allow users to specify which bands to include, rather than indexing everything in a multi-band raster. This is generally useful, and as a hypothetical workaround could allow a user to index the dataset without having to split it into multiple files; but this would require the user to perform multiple passes of the tool.