Allow overriding the function a MCState is sampled from

[inailuig]

Useful for e.g. implementing a importance sampled state as a subclass.

[PhilipVinc]

In the long run I'm not sure that's the best way forward but I'm fine with those changes and keeping them as internal changes (so not exposed)

I'd suggest adding a simple test for those properties or even that if you inherit MCState to declare a different sampler state it samples that one, just because if not it might break in the future.

Also add a comment somewhere in the file saying this is an internal way to override the distribution that is sampled.

[PhilipVinc]

@inailuig Do you want to add a test for better resiliency or should I merge as is?

[inailuig]

@inailuig Do you want to add a test for better resiliency or should I merge as is?

I think you can merge it as is, It will be tested once I add the importance sampling class.