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

for more information, see https://pre-commit.ci

[unittest/python] add test_struct

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

* optional: expose to-value converter for boost::none_t (and std::nullopt_t if possible)

+ add src/optional.cpp to expose none types
+ this allows to have None as a default arg value for functions (also, shows up in stubs)
+ also allows to have none_t return/argument type

* Run pre-commit

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

Co-authored-by: Justin Carpentier <justin.carpentier@inria.fr>

for more information, see https://pre-commit.ci

+ 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