docs: def fastas(self, download=False):
          """ Dict of filepaths for all fasta files associated with code.
  
          Parameters
          ----------
          download : bool
              If True, downloads the fasta file from the PDB.
              If False, uses the ampal Protein.fasta property
              Defaults to False - this is definitely the recommended behaviour.
  
          Notes
          -----
          Calls self.mmols, and so downloads mmol files if not already present.
          See .fasta property of isambard.ampal.base_ampal.Protein for more information.
  
          Returns
          -------
          fastas_dict : dict, or None.
              Keys : int
                  mmol number
              Values : str
                  Filepath for the corresponding fasta file.
          """
          fastas_dict = {}
          fasta_dir = os.path.join(self.parent_dir, 'fasta')
          if not os.path.exists(fasta_dir):
              os.makedirs(fasta_dir)
          for i, mmol_file in self.mmols.items():
              mmol_name = os.path.basename(mmol_file)
              fasta_file_name = '{0}.fasta'.format(mmol_name)
              fasta_file = os.path.join(fasta_dir, fasta_file_name)
              if not os.path.exists(fasta_file):
                  if download:
                      pdb_url = "http://www.rcsb.org/pdb/files/fasta.txt?structureIdList={0}".format(self.code.upper())
                      r = requests.get(pdb_url)
                      if r.status_code == 200:
                          fasta_string = r.text
                      else:
                          fasta_string = None
                  else:
                      a = convert_pdb_to_ampal(mmol_file)
                      # take first object if AmpalContainer (i.e. NMR structure).
                      if type(a) == AmpalContainer:
                          a = a[0]
                      fasta_string = a.fasta
                  with open(fasta_file, 'w') as foo:
                      foo.write(fasta_string)
              fastas_dict[i] = fasta_file
          return fastas_dict