extend_length
extend the search sequence such that
there will always be (prev) bases before the SNPs and
(after) bases after the SNPs.
extend_length(
overlaps,
position_reference,
genome_position,
prev,
after,
ori_string_start,
ori_string_end,
ori_snp_pos,
genome_max
)
Overlappings
the mapping of position in SNP matrix to reference genome
the position of the SNP in the reference genome
number of bases before the SNP included in the search string
number of bases after the SNP included in the search string
original starting point of search string
original ending point of the search string
original SNP position in search string
length of the reference genome
a list containing the new `string_start`, `string_end`, `snp_pos`, `snps_in_string`.