Problem: can't freely specify additional model parameters in project.xml

Solution: get additional params from nested param elements.

```xml
<model name = "my_model" script="my_model_generator.gsl">
  <param name="special_param" value="some_value" />
</model>
```
-> ```gsl -topdir:.. -script:my_model_generator.gsl -special_param:some_value -q my_model.xml```