def countGC(s, length_audit=True):
    if length_audit:
        assert len(s) == 30, "seems to assume 30mer"
    return len(s[4:24].replace('A', '').replace('T', ''))
GC content for only the 20mer, as per the Doench paper/code
dataset/ETHPy150Open MicrosoftResearch/Azimuth/azimuth/features/featurization.py/countGC