Add result_folder as object parameter

[RoiArthurB]

The second default parameter (result_folder) of COMOKIT's headless experiment :

<Experiment_plan>
  <Simulation experiment="SensitivityHeadless" finalStep="XXX" id="YYY" seed="ZZZ" sourcePath="/path/to/file.gaml" until="<optional>">
    <Parameters>
      <!-- default COMOKIT headless parameters -->
      <Parameter type="INT" value="0" var="idSimulation"/>
      <Parameter type="STRING" value="../batch_output/" var="result_folder"/>
      <!-- extra parameters here -->
    </Parameters>
    <Outputs/>
  </Simulation>
</Experiment_plan>

It should be settable in the GamaExploration object (which already handle all the XML parameters).