Numerical instabilities in Teubner Strey model

[wimbouwman]

We encountered this problem with the downloaded version of the executable SasView?.
When we try to calculate the intensities using for the parameters:
background =0
scale = 0.1
c2 = 4.9020e+04
c1 = -3.4265e+02
We obtain nonphysical sharp features.
We got at these values by taking the values for $d$ and $\xi$ we have obtained from SESANS measurements on oil water emulsions.
I could help to work with more the more physical parameters $d$ and $\xi$, as is done in ​http://dx.doi.org/10.1063/1.1377881

[wimbouwman]

Article with physical parameters Teubner Strey model in Eq(32)

[butler]

This is the same as #472 I believe. Since there is an attachement will leave here. But when fixed should close both tickets.

[butler]

with the 4.0 release being way late, we clearly don't have the bandwidth to deal with such a change now. Moving to +1

[butler]

Moving back to 4.0 as part of ticket #643 admonition to finalize parameter and model names so as to remain compatible in the future

[butler]

since this is essentially the same as ticket #472 owned by Andrew am assigning this one as well. If reassigned they should be reassigned together.

[ajj]

In caddb142198aa5e7856a0850214d72611c3b6ba8/sasmodels:

Updated and re-parameterised Teubner-Strey model.

Closes #472
Closes #530