Manual Redis to HDF5

Should we have an offline Redis to HDF5 converter for finished scans?

This would require refactoring of the Nexus writer code because anything could be missing at any point: keys could have been evicted from Redis, streams do not start from index 0, ... Currently exceptions are raised in these events. Or we don't refactor and incomplete scans simply not be converted.

@sole, @andy.gotz, @matias.guijarro