Helper function to parse a parameter from the INCAR.

def _parse_from_incar(filename, key):
    """
    Helper function to parse a parameter from the INCAR.
    """
    dirname = os.path.dirname(filename)
    for f in os.listdir(dirname):
        if re.search(r"INCAR", f):
            warnings.warn("INCAR found. Using " + key + " from INCAR.")
            incar = Incar.from_file(os.path.join(dirname, f))
            if key in incar:
                return incar[key]
            else:
                return None
    return None