Add support for conditional queries

[joy-xiaojizhang]

To use SPNs for conversational agents, we want to be able to evaluate the variables conditionally, i.e. not provide values of all variables for the evaluation. For example, in question answering, each training example consists of 2000 variables (10 words in each question/answer, length-100 word2vec representation of each word), whereas for testing, only 1000 variables corresponding to the question are provided.