Takes quantdata in a dict and header with quantkeys
    (eg iTRAQ isotopes). Returns dict of quant intensities
    with missing keys set to NA.

def get_quant_NAs(quantdata, quantheader):
    """Takes quantdata in a dict and header with quantkeys
    (eg iTRAQ isotopes). Returns dict of quant intensities
    with missing keys set to NA."""
    out = {}
    for qkey in quantheader:
        out[qkey] = quantdata.get(qkey, 'NA')
    return out