Metadata columns eligible for per-column PCA colouring

A column qualifies when, after dropping missing values for samples present in `sample_ids`, it has at least two distinct values, no more than `max_levels` distinct values (keeping legends readable), and every remaining value occurs at least `min_group` times (colouring by a level with one or two samples is not a meaningful group comparison).

Usage

pca_metadata_colour_columns(
  metadata,
  sample_ids,
  min_group = 3L,
  max_levels = 12L
)