LudvigOlsen/rearrr: rearrr 0.3.0

Breaking: Fixes bug where the shuffle_pairs and shuffle_members were kind of switched. Now shuffle_pairs changes the order of pairs while shuffle_members only shuffles the values of col.

Adds pipeline classes for piping transformations: Pipeline, GeneratedPipeline, FixedGroupsPipeline.

Adds transformation classes: Transformation, GeneratedTransformation, FixedGroupsTransformation. Used by their respective pipeline class.

Adds triplet_extremes() for arranging values in triplets with (highest, most middle, lowest), (second highest, second most middle, second lowest), etc. (Rearranger)

Adds order_by_aggregates argument to pair_extremes(). When using recursive pairing, this allows ordering the first num_pairings-1 pair identifier columns by the aggregate values instead of the identifiers.

Prepares package's tests for checkmate 2.1.0.