It can take any of the following values: ('strBin','logicBin','numBin').
'strBin' (String binary): each dinucleotide is represented by a string containing 4 characters(0-1). For example, AA = "0000"   AC="0001"   ...  TT="1111"
'logicBin' (logical value): Each dinucleotide is represented by a vector containing 4 logical entries. For example, AA = c(F,F,F,F)   AC=c(F,F,F,T)  ...    TT=c(T,T,T,T)
'numBin' (numeric bin): Each dinucleotide is represented by a numeric (i.e., integer) vector containing 4 numeric entries. For example, AA = c(0,0,0,0)   AC = c(0,0,0,1)  ...  TT = c(1,1,1,1)