Feature: - added `prepare` function to normalization, such that values can be computed over the entire histogram and passed specifically to the normalization function per sample or group - added `prepare_correlation_correction` to normalize data such the cell types with the largest correlation are superimposed between species. - added convenience functions for calculating mean, sum, standard deviation, and variance - added operations for +, -, and * Bugfix: - correct indexing when setting new values in a `KMerCollection` via the `subset_kmer_hist` function