`PhononWorkChain`: the `conv_thr` when using protocols refers to unitcell

[bastonero]

When using get_builder_from_protocol, the threshold per atom in

scf:
  meta_parameters:
    conv_thr_per_atom: 1.0e-15

will be used referring to the unitcell, not the supercell. This is due to the fact that we use straight the method of PwBaseWorkChain. A rescale would be desirable, e.g. by using the determinant of the supercell matrix.