object constraints in GPU engines do not work for multiple scans with shared object

[yijiang1]

In ptycho_solver.m, the loop "ll = 1:max(par.object_modes, par.Nscans)" for object constraints (e.g. apply_smoothness_constraint) will result in error if multiple scans share the same object. In this case, the number of object modes is less than par.Nscans. Need to find a way to determine the correct number of objects based on eng. share_object.