TODO

* Need unit tests

* stepwise: should have a "joggle" option.
* stepwise: A local (random?) search would be nice.

* ggmfit: should return number of iterations
* ggmfit: should be moved to gRbase

* Separate data processing for cmod (so that S needs not be calculated
  in each step in stepwise).

* Plot factor graphs

* testdelete -> testdrop testadd / testdrop -> testAdd / testDrop ?
* Should perhaps be clear from the names that these functions work on
   edges rather than on interaction terms?

* Could have addEdgeTest, dropEdgeTest, addTermTest, dropTermTest. 
  (Note: addEdge / removeEdge are standard generics.)

* Need decomposable.edge.deletion.test() function
  so as to get the degrees of freedom right with sparse data.

* Special functions/classes for handling decomposable models: 
  ddmod, dcmod, dmmod

* General imod function wrapper as common interface to the more
  specialized models.