Clarify the use of column roles for grouping features

[pat-s]

Currently there are multiple ways to do so

• via coords_as_features (legacy implementation), e.g. in SpCVCoords
• via $set_column_roles(), e.g. in SptCVCstf and others

Setting a column role for coordinates in SpCVCoords will not change the behaviour, i.e. coordinates stay out of the feature set (which is good).

This should be documented cleary in the mlr3book.

[pat-s]

We evaluate whwehter space and time should be stored in groups and handled natively by {mlr3}s grouping logic - together with the stratum column role.

The coordinates role might stay.

[pat-s]

• coords_as_features is a shortcut for task$set_col_roles(c("x", "y"), role = "feature")
• Using groups instead of space and time has been rejected internally as it would not work with the current mlr3 API