Worked on multifusion method.

BridgeStorage:
* Types are now templated.

FactorMapper:
* Factor and Storage are now stored as const references.
* Implemented method to map the variables and function values.

GraphicalModelBridge:
* Added method to translate the solution of the new graphical model back to the old one.

MultiFuser:
* The graphical model and the proposals are now stored as const references.
* The new graphical model and the graphical model bridge are returned as const references.