Changes to API

[nicrie]

To keep xeofs organized as we add more models in the future, I suggest we adjust the API with the next major release as follows:

• Single-set models in xeofs.single: EOF, OPA, GWPCA, EOFRotator, HilbertEOF, etc.
• Cross-set models in xeofs.cross: CCA (for 2 datasets), MCA, RDA etc., along with all the corresponding rotators, complex, and Hilbert models.
• Multi-set models in xeofs.multi: CCA (for more than two datasets) and possibly other models in the future, e.g. Common EOF analysis.

These models would each be based on different base classes, with nothing in common except the _BaseModel class for serialization and computing. This structure is already reflected in our documentation.

One specific scenario where this would help is with CCA. Right now, xeofs has a multi-set CCA implementation. With the current CPCCA class, it would be easy to offer a cross-set version of CCA with a more extensive API. The algorithm for cross-CCA is much simpler than for multi-CCA, where I'm still struggling to apply xr.apply_ufunc properly due to the need to explicitly know the number of input_core_dims and output_core_dims - which we generally don't know in advance for multi-set models. I'm sure there's a solution, but it might take time to find one. By splitting the namespaces, we could simply offer two versions: cross.CCA and multi.CCA, with different APIs depending on what we can ultimately implement for the multi-version.

Does this make sense to you @slevang ? Any concerns or thoughts on this?