reparameterize onion model

[pkienzle]

The existing onion model has two SLDs per shell, the inner and the outer. Tying the SLD at the interface between two shells therefore requires a constraint. If we instead given the SLD at the surface of each shell, we automatically tie the SLDs at the interface. Introducing an SLD jump will therefore require a new layer of zero thickness.

Since it will be easier for the user to introduce a jump layer than it is to tie the SLDs at the interface, then the new representation is preferred. If, however, the jump interface is by far the more common requirement, we should keep the existing interface.

[ajj]

Bumping to sasview 4.0. If we want to maintain backward compatibility in saved models, any parameter sets/names in 4.0 will be frozen for the foreseeable future.

[butler]

Not possible to get done for 4.0. If this becomes an issue at some point we should deal with it but will now move to +1. In order to ensure that old models can be loaded by not available to new need a deprecate old models functionality requiring some extensions to sasmodels…. Paul Kienzle has some thoughts on this.