docs: def arrays2wcxf(C):
      """Convert a dictionary with Wilson coefficient names as keys and
      numbers or numpy arrays as values to a dictionary with a Wilson coefficient
      name followed by underscore and numeric indices as keys and numbers as
      values. This is needed for the output in WCxf format."""
      d = {}
      for k, v in C.items():
          if np.shape(v) == () or np.shape(v) == (1,):
              d[k] = v
          else:
              ind = np.indices(v.shape).reshape(v.ndim, v.size).T
              for i in ind:
                  name = k + '_' + ''.join([str(int(j) + 1) for j in i])
                  d[name] = v[tuple(i)]
      return d