Refactor sum/product model

[pkienzle]

We should be able to define mixture models using something as simple as:

from sasmodels.sasview_models import mixture
Model = mixture('sphere', 'cylinder')

rather than large template that is currently produced by a sum model. This will allow us to evolve the sasview model interface without breaking custom mixture models store in the .sasview/plugin_models. Similarly for product models.

See #583 for more info.

May also consider making mixture/product models as something generated by the user as needed as part of the fit page rather than having to name every mixture that they are using in their fits.

Ideally this would be done as part of the 4.0.0 release so we do not need to support the existing sasview plugin model definitions indefinitely.

[pkienzle]

Generalize to:

Model = mixture('A + B', A='sphere', B='cylinder')

where the first value is an arbitrary expression and the keywords are the models to use in the expression.

[butler]

unfortunately ideal is not always practical. As discussed at meeting we don't have bandwidth to deal with for 4.0 release which is already way way late. Moving to +1

[butler]

As suggested by @richardh in ticket #1001 this is may be relevant to the beta approximation package so moving to that package.

HOWEVER … we should check but I think this may already have been done? If so we should close this ticket as part of 4.2 release.