+ currently produces a wonderful 36 errors :D

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Co-authored-by: Joris Vaillant <joris.vaillant@gmail.com>

Add example showing how to bind virtual classes, passing to overrides by reference, registering shared_ptr, and overriding factories and data classes from Python

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+ unittest: make bind_optional and test_optional.py into template files
+ cmake: instantiate boost and std optional tests using configure_file

+ default is boost::optional<T>
+ add test for boost::optional

+ create struct with fixed-size members, expose with setter

* tests we can expose std::vector<Matrix3d>
* test input and returning std::vector<Matrix> objects
* test specific functions for specific dims