Fix issues with uint8 conversion for float data in CopyVolume

[constantinpape]

The conversion to uint8 for float data in https://github.com/constantinpape/cluster_tools/blob/master/cluster_tools/copy_volume/copy_volume.py#L208-L214 is incorrect since it just normalizes based on the local statistics, which will lead to artifacts.
Instead we need to compute global min and max and pass it to the task for conversion without artifacts.