Reverse complement strings that represent stranded SBSs
RevcSBS96(mutstring)
Return the vector of reverse complements of the first 3 characters concatenated with the reverse complement of the last character, e.g. "AATC" returns "ATTG".
A vector of 4-character strings representing stranded SBSs in trinucleotide context, for example "AATC" represents AAT > ACT mutations.