On initialization, check if container group exists and read if available

[stephprince]

See related discussion here: #92 (comment)

After support for read is added, we could consider adding to Container.initialize() the ability to check whether the object already exists in the file and read it in if it is available. This would be relevant for creation of objects like Device and ElectrodeGroup, which may already exist in the file if created for a dataset of a different data type.