Add support for multiple-parameters

For #256.
Also removes deprecated 'getLearnedElement' method in unit tests
Fix for issue #288
Also fix for issue #288
Merge pull request #289 from bruttenberg/DEV2.4.0

Fixes for issue #288
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
Merge pull request #286 from mhoward2718/DEV2.4.0

Add support for multiple-parameters
Merge pull request #1 from p2t2/DEV2.4.0

update fork from Dev2.4.0