docs: def get_meshes_fld(step, var):
      """Return scalar field along with coordinates meshes.
  
      Only works properly in 2D geometry.
  
      Args:
          step (:class:`~stagpy.stagyydata._Step`): a step of a StagyyData
              instance.
          var (str): scalar field name.
      Returns:
          tuple of :class:`numpy.array`: xmesh, ymesh, fld
              2D arrays containing respectively the x position, y position, and
              the value of the requested field.
      """
      fld = step.fields[var]
      if step.geom.twod_xz:
          xmesh, ymesh = step.geom.x_mesh[:, 0, :], step.geom.z_mesh[:, 0, :]
          fld = fld[:, 0, :, 0]
      elif step.geom.cartesian and step.geom.twod_yz:
          xmesh, ymesh = step.geom.y_mesh[0, :, :], step.geom.z_mesh[0, :, :]
          fld = fld[0, :, :, 0]
      else:  # spherical yz
          xmesh, ymesh = step.geom.x_mesh[0, :, :], step.geom.y_mesh[0, :, :]
          fld = fld[0, :, :, 0]
      return xmesh, ymesh, fld