cmake: update FindVLE and FindGVLE

- related to commit 550d21096be9c8a0d965b3617a11a707c697be03 of vle
- related to commit 2bc632b82032f9caa78cd66a30dc740e81de3f30 of vle
- related to commit d1a8cfd63cc6610a1d447300f2ef2f949f921909 of vle
- related to commit 39f3bdca3e7cf944494b6f8dc939c69f7f9c6903 of vle
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)
forrester: update the Conditions

The variables initialisation system has been updated. Atomic double
port parameter are used in order to ease analyses with R. Each
Variables will have a port parameter with its name and its initiial
value.
(closes #44)
forrester: fix the drawing area refreshment

The pixmap needed to be resized at the wright time, followed by a
queueRedraw.
(closes #46)