Define consecutive column blocks as feature sets

Convenience constructor for the common case where columns are arranged as consecutive blocks (e.g., low/mid/high/sem PCs concatenated).

Usage

blocks(...)

...: Named integers giving number of columns per set (must sum to `ncol(X)`).

An object of class `feature_set_spec_blocks`.

`blocks()` does not touch any data; it only declares how columns should be interpreted when passed to `feature_sets()`.

spec <- blocks(low = 100, mid = 100, high = 100, sem = 100)