fix profiling/LotkaVolterra.vpz model

The name of the package for the dynamic used was wrong.
all: fix string comparison in CMakeLists.txt

Keyword EQUAL was used to check the CMAKE_BUILD_TYPE value and that did not
work. It is now replaced by STREQUAL.
differential-equation: add output_period parameter

Output of variables was performed at each time step of integration.
To reduce the number of outputs, the parameter output_period (default to 1)
allows to perform outputs every output_period integration steps.
diff-equ: remove PDF from source tree
diff-equ: add vle.output in build depends
difference-equation: add a constructor field

The modeling plugin has now a new field that enable to add custom code
into the constructor method. From now on, this way, we can define
custom parameters.
(closes #50)
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.
decision: fix the activity dialog

- When the maxfinish or the minstart is not defined, it does stay
undefined.
- maxfinish has to be bigger than minstart before to leave the dialog.
- maxfinish an minstart have to be an mpty string or an integer or
human date before to leave the dialog.
- when switching from relative date to absolute, the dialog tries to
translate.
(closes #71)
decision: add a relative date to year concept

The plan file accepts relative calendar date like this "+YYYY-MM-DD"
where +YYYY stands for the number of the year. For exampple "+0-3-10"
stands for the third of february of the year n of the load date of the
plan. And "+1-11-5" stands of the fifth november of the year n+1. The
29 february is not allowed. Reklative numerical date are still
allowed.  Inside the modeling plugin. The Activity Dialog window allow
this new feature.