Convert a vector of partition indices into meta cluster labels
Value
A character vector that expands the split_vector into an nrow-length sequence of ascending letters of the alphabet. If the split vector is c(3, 6) and the number of rows is 8, the result will be a vector of two "A"s (up to the first index, 3), three "B"s (up to the second index, 6), and three "C"s (up to and including the last index, 8).