code: def step_group(k):
      
  
      cbpdn_xstep(k)
      if mp_xrlx != 1.0:
          cbpdn_relax(k)
      cbpdn_ystep(k)
      cbpdn_ustep(k)
      ccmod_setcoef(k)
      ccmod_xstep(k)
      if mp_drlx != 1.0:
          ccmod_relax(k)
docs: Do a single iteration over cbpdn and ccmod steps that can be
      performed independently for each slice `k` of the input data set.