Get three-letter aa code if possible. If not, return None.

    If three-letter code is None, will have to find this later from the filesystem.

    Parameters
    ----------
    aa_letter : str
        One-letter amino acid code.

    Returns
    -------
    aa_code : str, or None
        Three-letter aa code.

def get_aa_code(aa_letter):
    """ Get three-letter aa code if possible. If not, return None.

    If three-letter code is None, will have to find this later from the filesystem.

    Parameters
    ----------
    aa_letter : str
        One-letter amino acid code.

    Returns
    -------
    aa_code : str, or None
        Three-letter aa code.
    """
    aa_code = None
    if aa_letter != 'X':
        for key, val in standard_amino_acids.items():
            if key == aa_letter:
                aa_code = val
    return aa_code