naming of functions

[dramanica]

We need a bit of clean up of names. The suggested logic is that:

• 'indiv' is used as a prefix for functions applied at the individual (row) level, e.g. indiv_missingness().
• 'loci' is used as a prefix for functions at the locus (column) level, e.g. loci_missingness().
• 'gt' is used for functions that apply to the whole gen_tibble, e.g. gt_pca

We need to think about functions that make pairwise comparisons (e.g. ibs, king, pairwise_pop_fst). One option is to use the prefix between_. However, this can be ambiguous, as a between_ function would return a matrix that could be n x n individuals, or m by m populations. One option is to say that between_ functions return a matrix of comparisons between individuals, and that between_pop_ is used for populations. So, we would have between_ibs or between_king and between_pop_fst.

[dramanica]

Since between is a dplyr function, we will switch to pairwise_*.