Implement method to set parameters equal without fixing them

[DavAug]

Add method unify_parameters to ReducedModels.

• Implement unify_parameters which takes a list of parameter names and sets them equal.
• Once parameters are set to be equal they act as if they were one parameter
• Optionally a parameter name can be provided that will be used to identify the "new" unified parameter. If no name is provided the first parameter is used as the parameter.
• The location in the parameters list is the first parameter in the list.
• To unset parameters the method can be called on just the unified parameter.