Check Pybind version

[sjunges]

Based on a discussion in moves-rwth/pycarl#15 (comment) we should check the PyBind11 version that pycarl uses and compare it to the one we are using for stormpy. While it may work without issue, at least providing a warning may be essential to prevent strange errors.

[volkm]

I fully agree.
The technical question is how to get the pybind version? While pybind has the variable pybind11_VERSION (see here), using add_subdirectory opens a new scope such that one cannot access this variable from outside.
One possibility might be to use find_package which needs an installation of pybind though.
Or we build a python function which accesses the version information available in pybind11.h. However, in that case the check can only be performed after compilation is finished.

[sjunges]

See also #86

[volkm]

The check was integrated by PR #86.