Support more than one initial state

Modest does currently not support more than one initial state. This means that initial state restrictions using disjunctions or shrinking variables to a certain range are not allowed. Therefore this issue is in some sense related to #44. A workaround would be to add a dummy initial state with tau transitions to all other real initial states. But since PRISM supports this feature, all JANI translations from such PRISM models will result in Jani files not directly (without a preprocessing step adding the dummy initial state) supported by Modest.