Download pybind11 via CMake instead of shipping it

[volkm]

Download pybind11 in the configure step such that it is available for add_subdirectory.
Should best be merged simultaneous to the similar PR in pycarl moves-rwth/pycarl#15.

[volkm]

Will also update pybind to version 2.10.0 and should fix #68.

[sjunges]

I think that ultimately, it would be good if we could set the pybind version as an option. That way, if someone depends on a particular pybind version, it is easier to fix. Not urgent though.

Otherwise: LGTM and many thanks

[volkm]

One can already define the version in include_pybind11.cmake. Setting it in CMake and from Python setup.py should be easy to implement. Setting the version should however be an expert option, because pybind updates might require code adaptions.

[sjunges]

As it currently must be the same as pycarls version, does it make sense to take the pycarl version as default?

[volkm]

The recent changes allow to set the pybind version to use. As default we are using the pybind version exported by pycarl (see moves-rwth/pycarl#15). Note that this requires to import pycarl during the build process and pycarl was added to setup_requires.

[volkm]

Tests are currently failing, because moves-rwth/pycarl#15 is not used yet.

[sjunges]

LGTM