Added copy constructor for Args.

	  * src/Args.cc: Here.
	  * src/Args.hh: Here.
Added an engine templated by the data term to make switching between DT easy.

	  * src/Engine.hh: Here.
	  * src/Engine.hxx: Here.
	  * src/main.cc: Use it.
Modified the way Us in LogDataTerm is computed (still experimental).

		 * src/LogDataTerm.hxx: Here.
Added an option to switch dataterm in parameters.

 * src/Args.cc: Here.
 * src/Args.hh: Here.
 * src/Engine.hh: Changed a bit the prototype.
 * src/Engine.cc: Changed a bit the prototype.
 * src/main.cc: Use it.