Support IUPAC ambiguity codes in primer sequences

[ressy]

CHIIMP should support IUPAC codes for ambiguous nucleotides in primer sequences in order to recognize multiple sets of possible primers for a single locus.

[ressy]

This is probably a good opportunity to move away from character vectors to support these sorts of features. Biostrings supports ambiguous matches via IUPAC codes easily:

> Biostrings::vmatchPattern(
+     "RCTG",
+     Biostrings::DNAStringSet(c("ACTGAAAATCAA", "GCTGAGGCTCAA", "TCTGAAAATCAA")),
+     fixed = FALSE)
MIndex object of length 3
[[1]]
IRanges of length 1
    start end width
[1]     1   4     4

[[2]]
IRanges of length 1
    start end width
[1]     1   4     4

[[3]]
IRanges of length 0