cal_met_snp
is used to calculate the metric at each position
cal_met_snp(position, metric, seqc, prepend_position = c(), ...)
position to check
either `simpson` or `percent`
list of sequences, either passed directly from
process_allele
or read_fasta
or equivalence
is the position to be added to the
other parameters as needed
return the value at that position, as well as base pattern for next iteration.