docs: def get_rprof(step, var):
      """Extract or compute and rescale requested radial profile.
  
      Args:
          step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
              instance.
          var (str): radial profile name, a key of :data:`stagpy.phyvars.RPROF`
              or :data:`stagpy.phyvars.RPROF_EXTRA`.
      Returns:
          tuple of :class:`numpy.array` and :class:`stagpy.phyvars.Varr`:
          rprof, rad, meta
              rprof is the requested profile, rad the radial position at which it
              is evaluated (set to None if it is the position of profiles output
              by StagYY), and meta is a :class:`stagpy.phyvars.Varr` instance
              holding metadata of the requested variable.
      """
      if var in step.rprof.columns:
          rprof = step.rprof[var]
          rad = None
          if var in phyvars.RPROF:
              meta = phyvars.RPROF[var]
          else:
              meta = phyvars.Varr(var, None, '1')
      elif var in phyvars.RPROF_EXTRA:
          meta = phyvars.RPROF_EXTRA[var]
          rprof, rad = meta.description(step)
          meta = phyvars.Varr(misc.baredoc(meta.description),
                              meta.kind, meta.dim)
      else:
          raise UnknownRprofVarError(var)
      rprof, _ = step.sdat.scale(rprof, meta.dim)
      if rad is not None:
          rad, _ = step.sdat.scale(rad, 'm')
  
      return rprof, rad, meta