Allow MultivariateParameters to be set within ParameterValues

[NicolaCourtier]

Align the set-up of MultivariateParameters with Parameters by allowing the user to mark the relevant parameters within a typical pybamm.ParameterValues object before passing it to a simulator. At the moment, the parameters attribute of a simulator has to be overridden after initialisation by MultivariateParameters and these new parameters are not currently shared when the simulator is passed to a problem - see this example.

Note that, whenever two sets of parameters are joined together, any duplicate parameters need to be discarded. Other checks will be needed to ensure that if there is a multivariate distribution then there are no other optimisation parameters.