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 #2 from p2t2/DEV2.4.0

Dev2.4.0
Minor changes to shared BP code.
Merge pull request #294 from bruttenberg/DEV2.4.0

Dev2.4.0
Updated FF, added new ElementSampler to algorithms which will sample an
element using Forward samplng but implements an algorithm interface
Added a contains function to the FactorGraph interface
Added small fix to GeneralizedEM to get it to work with BP (must clear
factors inside EM instead of inside BP since BP is used for many things
now).

Added a new Apply interface to Forward, so if you give it an element
instead of a universe, it will forward sample the element.

Small fixes to importance sampling tests to enable better error bounds
Merge pull request #3 from p2t2/DEV2.4.0

Merge pull request #294 from bruttenberg/DEV2.4.0
Merge pull request #295 from bruttenberg/DEV2.4.0

Dev2.4.0
Fix for issue #297

All atomic elements in the libraries will now return 0 if you request
the density of a value outisde of its support
Merge pull request #4 from p2t2/DEV2.4.0

Merge pull request #295 from bruttenberg/DEV2.4.0