0.5.5

• determining lattice symmetry and (psuedo-)merohedral twinning laws
• added GroupOps::derive_symmorphic() and GroupOps::add_inversion()
• getting change-of-basis in the Niggli reduction
• added transform_to_assembly() – it does a bit more than make_assembly()
• removed Op::negated()
• python: add bindings to UnitCell.orth and UnitCell.frac
• additional options in gemmi-mtz2cif and gemmi-cif2mtz
• bug fixes (in Ccp4::set_extent(), merge_atoms_in_expanded_model(), status column in mtz2cif)