Add ColSpec support for more transforms

[juliohm]

EigenAnalysis, PCA, SDS, DRS, ProjectionPursuit could all leverage the ColSpec for consistency.

[juliohm]

As discussed on Zulip, the benefit of this feature does not surpasses the amount of refactoring work needed. I will close for now, and we can reassess in the future if necessary. Currently it is easy enough to select columns with Select and then forward the transform to PCA, SDS, ...