Allow separate parameters or blocks of them to use different samplers

[ben18785]

Not sure how easy this is to do in Pints but will likely be very useful for bigger models, with correlated parameters.

[MichaelClerx]

@ben18785 is this a duplicate of #646 ?

[MichaelClerx]

If you could combine this with #646 it would probably make a good project.

I think it'd largely be a feature of the MCMCController (unless you wanted to allow e.g. ABC or nested sampling for some blocks?), so a possible road map would be:

• work out a single ask() call (in a script), that can use multiple MCMC samplers on the same LogPDF. The MCMCSampler class doesn't know about the problem it's optimising (only it's n_parameters) so that should be fairly straightforward
• make a call to tell --> now it get's a bit trickier, as some samplers could return None here?
• work out what to do in this case
• wrap it up in a loop, see if it works
• depending on how much code it is, add it to MCMCController, or write a separate controller