allow multiple array dispersity parameters

[pkienzle]

The current implementation of fitpage (sasview 4.0) keeps a single values and weights vector that is shared by all array dispersion parameters. This means only one parameter can have array polydispersity.

Rather than keeping polydispersity information in the FitPage attributes, query and update it directly from the sasview_model. That way the model distribution parameter can maintain separate array distributions for each parameter.

PageState and the XML representation will need to be updated so that each distribution can have its own parameters. You may also want to rearrange the hierarchy so that the parameter contains the distribution rather than associating parameter and distribution through parameter name.

[butler]

This would indeed be a good code cleanup/refactor task but not likely to be done in time for 4.1 to be released so moving to next +1 for now