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