This is an explicit fix for issue #224. This still shouldp probably be
addressed in a more complete fashion (sharing factor creation code), but
right now this fix is needed to support issue #265
Removed dead code
Merge remote-tracking branch 'upstream/DEV2.4.0' into DEV2.4.0
Refactored some code in factored frontier that runs the inner BP loop
into its own trait so it can be shared between other algorithms that use
BP as an inner loop (e.g., particle BP).
Merge pull request #291 from bruttenberg/DEV2.4.0

Fixes for #224