Single-color decompression for `TreeSetColoringResult`?

[gdalle]

I didn't implement it because I'm not even sure it is possible. @amontoison what do you think?

[gdalle]

According to Alexis, the best we can do is decompress tree by tree. This requires more than one column of B, and up to every column in the worst case. So it's not worth adding to the API: if memory is an issue, use :direct decompression.