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[unittest/python] add test_struct

+ do not test slice for std::array<Eigen..., k>

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

+ 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