Revise stormpy modules

[volkm]

• Add an underscore suffix to all pybind libraries, e.g. _storage instead of storage. This helps distinguishing between the Python module storage and the pybind library _storage. Some adaptions were required but nothing should change on the outside.
• Added new module stormpy.exceptions and added type information to docstring
• Revised documentation for stormpy.info. I tried making the functions storm_version(), etc properties, but this lead to issues because the property decorator is aimed at classes and not modules.
• Generate API documentation for stormpy.pomdp
• Use sphinx decorator for deprecation. Deprecations now require a version and I opted to use the last version 1.10.0. Technically the deprecation was only introduced afterwards, but otherwise we need to reference a future version without knowing what version number we will actually have.
• Our Sphinx documentation generates documentation also for imported members (to include the pybind libraries). I needed to explicitly exclude the deprecated import as it would occur in the documentation otherwise. We have to keep an eye on future imports which we also might need to exclude.

[sjunges]

LGTM!