Describe the bug
The Gaussian distribution is the only distribution available for Distribution of thickness in the core_multi_shell model.

To Reproduce
Steps to reproduce the behavior:

1. In the fit panel, choose sphere, core_multi_shell,
2. Check Option > Polydispersity, then click on the Polydispersity tab,
3. See error: no drop-down menus for thicknesses.

Expected behavior
A drop-down menu in Function for each thickness.

Screenshots

Note
For the remaining drop-down menu (thickness2, not radius), a choice of distribution can lead to the following error:

12:58:47 - ERROR: Traceback (most recent call last):
  File "sasview/src/sas/qtgui/Perspectives/Fitting/FittingWidget.py", line 3240, in <lambda>
    func.currentIndexChanged.connect(lambda: self.onPolyComboIndexChange(str(func.currentText()), i))
  File "sasview/src/sas/qtgui/Perspectives/Fitting/FittingWidget.py", line 3316, in onPolyComboIndexChange
    self.kernel_module.set_dispersion(param.name, self.disp_model)
  File "sasmodels/sasmodels/sasview_model.py", line 816, in set_dispersion
    raise ValueError("%r is not a dispersity or orientation parameter"
ValueError: 'thickness[n]' is not a dispersity or orientation parameter

SasView version (please complete the following information):

• Version: 5.0.6

Operating system (please complete the following information):

• OS: Linux