Automatically determine layer type (and channels?)

[davidackerman]

If we implement postprocessing, the output could be uint64 something. Currently we just assume everything is uint8, but presumably predictions or other arbitrary processing could be floats or other uint types. It might be nice to run through a sample block once; this would be good both for testing and getting the output type. This could also be useful for determining channels automatically?

[mzouink]

• dtype based on postprocessing steps is there now.

Can you point me to the example code for naming the channels in neuroglancer instead of 1..2..