Use consistent naming for ValueType

[volkm]

The stormpy datastructure need different names depending on the underlying value type.
I think the current proposal is to use the following suffixes:

• `` (empty) or Double
• Exact
• Parametric
• Interval
  We can then use templated functions with the suffix to instantiate all value types.
  An example can be seen for the models

This issue keeps track of the progress to use consistent naming.