code: def get_aa_code(aa_letter):
      
      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
docs: 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.