Cxx/Cells/_assign: a little safer in-place changes handling

zUtil_Cxx/cell_operations/internal_cell_div_assign.cpp

zUtil_Cxx/cell_operations/internal_cell_exponent_assign.cpp

zUtil_Cxx/cell_operations/internal_cell_non_neg_assign.cpp

zUtil_Cxx/cell_operations/internal_cell_prod_assign.cpp

..._Cxx/cell_operations/internal_cell_prod_scalar_assign.cpp

...ell_operations/internal_cell_single_sinogram_to_blobs.cpp

zUtil_Cxx/cell_operations/internal_cell_sqrt_assign.cpp

zUtil_Cxx/cell_operations/internal_cell_sub_assign.cpp

...l_Cxx/cell_operations/internal_cell_sub_scaled_assign.cpp

...cell_operations/internal_cell_sum_FISTA_scaled_assign.cpp

...x/cell_operations/internal_cell_sum_FISTA_scaled_copy.cpp

...rations/internal_cell_sum_FISTA_scaled_non_neg_assign.cpp

zUtil_Cxx/cell_operations/internal_cell_sum_assign.cpp

...l_Cxx/cell_operations/internal_cell_sum_scalar_assign.cpp

...l_Cxx/cell_operations/internal_cell_sum_scaled_assign.cpp

zUtil_Cxx/include/internal_cell_defs.h