Support for discounted properties in MDPs and DTMCs

[AlexBork]

This PR adds support for checking cumulative and total reward properties with a given discount factor on MDPs and DTMCs.

• Extends the parser to allow discounted properties, proposed syntax is R=? [Cdiscount=_factor_]
• Introduces helper classes for checking discounted properties
• Extends the respective PCTL model checking classes with support for discounted cumulative and total reward properties, implemented analogous to the existing undiscounted implementations