Add module for combining SasView models with reflectivity data

[hoogerheide]

Sometimes reflectivity data have features that look like they might come from small angle scattering. Usually this means the sample needs to be re-made, but there are cases where this might actually be useful data, and other cases where the sources of the small angle scattering can otherwise not be removed.

This PR is for an Experiment-like object that combines SANS and reflectivity data into a single curve, and exposes the parameters of the sasmodels SANS model as Bumps parameters. The model specification includes the SANS model name, a dictionary of parameter values, and a transverse angular resolution dtheta_l which is used to smear the SANS data.

A custom decomposition plot into I(q) and R(q) is registered as well.

[hoogerheide]

This is an example script. You can put it any dummy data for this to work.

test_sas_script.py

[hoogerheide]

This is an example script. You can put it any dummy data for this to work.

test_sas_script.py

Now look at the tiox_dopc_sas.py example in the repo