code: def step_indices(group_idx):
      
      ilen = step_count(group_idx) + 1
      indices = np.empty(ilen, int)
      indices[0] = 0
      indices[-1] = group_idx.size
      inline(c_step_indices, ['group_idx', 'indices'], define_macros=c_macros, extra_compile_args=c_args)
      return indices
docs: Get the edges of areas within group_idx, which are filled 
          with the same value