First version of save function + unit tests

Finished first version of qim3d.io.save() + unit tests. I am not sure what self.verbose is intended to do, so for the moment, I have just left it unused in init()

qim3d/io/__init__.py

 from .load import DataLoader, load, ImgExamples
-from .save import save
+from .save import DataSaver, save 
 from .sync import Sync
 from . import logger
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