Set some observations for the sampler

[gowthamnatarajan]

The sampler class generates data randomly from the graph. Is there a way I can specify values for some variables (like some variables are observed) and then generate samples for the remaining variables.

[andresmasegosa]

That would be a nice addition. Actually, we have something like that in the Importance Sampling algorithm implemented by @dariorlual. We will try to add it to the core package.