Remove (or update) dependency to pydap

[TonioF]

Currently, the toolbox is pinned to use exactly version 3.3 to pydap. As this is likely to cause issues in the future, an alternative solution should be used. Ideally, we would make use of the opendap implementation of fsspec, as this is in line with the function of xcube. It might make sense to create an xcube accessor.