AT/GC Ratio
y = btd_at2gcratio(x)
String of single-letter codes specifying a DNA/RNA sequence
Ratio of AT/GC (decimal number).
Computes the ratio between AT and GC
y = btd_at2gcratio("ACTCTAGCTACT")
--> y = btd_at2gcratio("ACTCTAGCTACT") y = 1.4