docs: def _scalar2array(d):
      """Convert a dictionary with scalar elements and string indices '_1234'
      to a dictionary of arrays. Unspecified entries are np.nan."""
      da = {}
      for k, v in d.items():
          if '_' not in k:
              da[k] = v
          else:
              name = ''.join(k.split('_')[:-1])
              ind = k.split('_')[-1]
              dim = len(ind)
              if name not in da:
                  shape = tuple(3 for i in range(dim))
                  da[name] = np.empty(shape, dtype=complex)
                  da[name][:] = np.nan
              da[name][tuple(int(i) - 1 for i in ind)] = v
      return da