idx2 and OpenViSUS integration

[hoang-dt]

Current status:

idx2 dataset can be uploaded and retrieved from object storage.

Limitations

@scrgiorgio: can you upload your powerpoint slides with the TODO list?

• Small chunk size (see [OpenViSUS and idx2 integration] idx2 chunks being too small for cloud #8)

TODO

@scrgiorgio: can you add instructions for the following?

• How to generate an idx2 dataset that can be uploaded to the cloud
• How to upload such a dataset to the cloud?
• How to retrieve such a dataset from visusviewer/OpenViSUS python?