Support for interval models

[sjunges]

With #409, storm has now preliminary support for intervals. This is by no means stable. Some important features that we are still missing are:

• Explicit support for solving on DTMCs -- the DTMCs must be declared as MDPs
• Support for rewards (this has been partially added in iMC changes outside of storm-pars #627 and iMDP: allow reachRewards from outside & limited policy extraction support #741)
• Bounded reachability properties
• Topological VI
• Policy extraction
• Convenience functions to remove uncertainty (e.g., by taking the 'center').
• support for robustness in formulas
• Generating interval models from PRISM/JANI
• Exact intervals

[volkm]

I think at least support for parsing interval models from Prism/JANI would be nice to have soonish.