Remove slack parameter from GISModel

The support for a slack parameter inside the model should be removed from the GISModel class. Only old models which have been trained prior maxent 3.0 can have such a slack parameter or correction constant. The training code since maxent 3.0 always sets the correction constant to 1 to support backward compatibility with old models.
Since the removal will break backward compatibility it should be aligned with the opennlp-ml redesign.

The support to train models with a slack parameter has been completely removed in OPENNLP-165.