Support for interval models

[sjunges]

With https://github.com/moves-rwth/storm/pull/409, storm has now preliminary support for intervals. This is by no means stable. Some important feature that we are still missing are:

• [ ] Explicit support for solving on DTMCs -- the DTMCs must be declared as MDPs
• [ ] Support for rewards
• [ ] Bounded reachability properties
• [ ] Topological VI
• [ ] Convenience functions to remove uncertainty (e.g., by taking the 'center').
• [ ] support for robustness in formulas
• [ ] Generating interval models from PRISM/JANI