Remove prune_events from BranchSPN and make ProductSPN check for conjunctions first

[fsaad]

Rather than iterate through all 2^m subsets, first compute the probabilities of the m conjunctions and immediately return 0 if each conjunction in the union has probability zero, which removes the duplicate computation inside prune_events.

[fsaad]

Implemented in 4ac7d47 for logprob---condition is more complicated, since finding the disjoint union is often more expensive than the linear pruning, plus we already memoize those computations