Force custom program termination
On run call, it is possible to specify, which steps should be executed. Because every steps depends on its previous steps, it shouldn't be possible to call training only. But on the other side, it should be possible to stop after preprocessing or any other step. This is required to run different parts of an experiment on different partitions (see #104). If a termination step is given, only all precursory steps and itself are executed. Progress is saved locally (anyway because of #102).