Remove ETH-specificity

[znicholls]

Many data assumptions rely on the ETH data structure. At some point we should allow the users to hold the data however they want, load it how they want and then have interfaces into which they pass the data.

[mathause]

I think we are on the way there. By disentangling the data loading from its preparation (i.e. calculating anomalies etc.) this becomes easier. Once we defined the way our input data needs to look (e.g. datatree) users can create this structure and go.

Still we might add a way to load data from cmip6-ng directly, for convenience. However, we could potentially also add wrappers from ESMValTool or intake (bat that is more on the long term wish list).