multilayer vesicle doesn't define effective radius

[pkienzle]

Documentation for multilayer vesicle says that it uses the outer shell radius as the effective radius when P(Q)*S(Q) is applied, but ER() is not defined, so no structure factor can be applied.

This feature was available in the MultiShellModel? in 3.1.2.

Note that the equation docs can be improved for this model.

Also, it should use n_shells rather than n_pairs since there is one fewer solvent layer than shell layer.