vle.forrester: fix the timeStep Button

Also fix the Include button. Connection signals were destroyed much to
soon.
vle.forrester: fix the time step management

For the compartment dialog and the pure time step dialog, control and
visualisation of doubles have been improved.
vle.forrester: add an output period entry

In order to use the new output_period parameter available, an entry
has been added.
diff-equ: use vle.output instead of vle.output (=1.3)

To avoid VleUtils.cmake script error.