Describe the bug
Loading a project containing some constraints works as expected for the constrains related to the main model parameters, but not for those related to polydisperse parameters.

To Reproduce
Steps to reproduce the behavior:

1. Load 2+ datasets
2. Go to Constrained or Simultaneous Fit
3. Define several constraints, including at least one for a PD parameter, e.g.:
   M2.radius = M1.radius
   M2.radius.width = M1.radius.width
4. Save the project
5. Load the project: Only the constraint for M2.radius appears in the list of constraints. The one for M2.radius.width needs to be rewritten.

Expected behavior
Loading the project should also load the constraints defined for PD parameters.

SasView version (please complete the following information):

• SasView 5.0.6? Tested in ESS_GUI_constrain_branch to be merged, as before it was not even possible to define constraints for PD parameters.

Operating system (please complete the following information):

• Windows 10