Sets the encoding variable according to the text passed

    :param sval: text specification for the desired model

def SetEncoding(sval):
    """Sets the encoding variable according to the text passed

    :param sval: text specification for the desired model
    """
    global encoding
    s=sval.lower()
    if s == "additive":
        encoding = Encoding.Additive
    elif s == "dominant":
        encoding = Encoding.Dominant
    elif s == "recessive":
        encoding = Encoding.Recessive
    elif s == "genotype":
        encoding = Encoding.Genotype
    elif s == "raw":
        encoding = Encoding.Raw
    else:
        raise InvalidSelection("Invalid encoding, %s, selected" % (sval))