Make Sampler a field of DefaultContext (renamed to SamplingContext)

[mohdibntarek]

Currently we have 2 arguments for changing the behavior of assume and observe, context and sampler. I think:

1. Sampler belongs in a field in DefaultContext, perhaps renaming it to SamplingContext,
2. SampleFromPrior should always overwrite the values in VarInfo much like SampleFromUniform in Fix SampleFromUniform #78, and
3. ComputeLogpContext can be another context which encompasses LikelihoodContext, PriorContext and the current behavior of DefaultContext with SampleFromPrior.

I always found it confusing the SampleFromPrior doesn't actually sample anything if VarInfo already has values.

[yebai]

@penelopeysm is this now fixed?

[penelopeysm]

Totally missed this! But yes, (1) and (3) are fixed.

SampleFromPrior is still misleadingly named as it is a no-op if variables already exist in VarInfo. I have found this v v confusing myself too. But I think we can open a separate issue for this

[penelopeysm]

#859