forrester: update terminology

Little changes have been made to the terms used for the GUI.
(closes #17)
packagevle-1.1.0-beta1
all: update CMakeLists.txt without GVLE

The decision, difference-equation, fsa and petrinet must be use without the
graphical user interface ie. GVLE and its dependencies. (closes #38).

- We fix the variable HAVE_GVLE for all package.

- We update the FindVLE and FindGVLE from VLE repository.
forrester: update the observables generator

Observables are not removed each time a model is regenerated.  If the
observable exist it is keeped, and if a observable is not any more
needed it is removed.
(closes #36)