Change interface of logpdf to take a Python dict not Event

[fsaad]

Using an Event of the form spn.logpdf(X << {.5}) is the wrong abstraction, since a p.d.f is a function defined on the range (outcome space / codomain) of the random variable, not the sigma algebra. That functions on the former can be queried using an interface to the latter---i.e., "X is contained in a singleton set"---is an abuse.