Simplify DBN XADD w.r.t. state-invariants

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In reference to the discussion in issue #2, we should simplify XADDs according to state-invariants.

This can be done as follows:

1. Compile each state-invariant into its own XADD (exactly the same way as cpf expressions are currently compiled). Satisfaction of the state-invariant should lead to leaf 1. Dissatisfaction should lead to leaf NaN.
2. The compiled cpf XADD should be replaced with a multiplication of itself with all state-invariant XADDs.

Leaves in the resulting cpf XADDs will then lead to a leaf value of NaN if that state is unreachable under the state-invariants. (I.e., if the user tries to evaluate the cpf in an illegal state, there has to be some way for the XADD to indicate that this is an illegal state w.r.t. the state-invariants and hence cannot be evaluated.)