Add empty and common steps in HLT CF menu
Main changes in the CF builder to manage chains that share only parts of sequences. If two chains have a common step, but rest is different, the CF sequence is duplicated because one needs a new Filter+new Hypo (duplicated as well). This makes the graph more complicated to debug. See attached diagram.
Some changes in the CF builder to include the empty steps. Empty steps are managed as steps with one Filter Alg only, so successive step receives the output of the FilterAlg, instead of the Hypoalg. Some changes are needed in the Dot functions, to envisage such cases. The emu_step test has been made more modular, to allow preparing more complex and large menus in the unit tests. Added more use-cases in this test. Covering use cases in ATR-19600. Many ref files are updated, need to ask the experts if reasonable results are reached with these changes.