* 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

* [pre-commit.ci] pre-commit autoupdate

updates:
- [github.com/psf/black: 22.12.0 → 23.1.0](https://github.com/psf/black/compare/22.12.0...23.1.0)

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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



---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

remove useless include
remove comment between includes (not sorted by pre-commit anymore)

core/std-vector.hpp
* fix using-ctor for older cpp versions
* fix warning about conversion

& update its docstring

* fix includes
* comment out edge cases

unittest/eigen_ref.cpp: asConstRef should take input as Ref<const T>

eigen-allocator: try a fix for mutable Eigen::Ref from Python array with "wrong" layout

* trick transposes pyArray to get the right layout :)

std-map/std-vector: add mention to file heading

* bp::extract will construct an Eigen::Ref on the fly as an rvalue using the registered converter
* move overload_base_get_item_for_std_vector from pinocchio to here -- changes behaviour of __getitem__
* redef exposeStdVectorEigenSpecificType to overload the boost vector_indexing_suite __getitem__
* move exposeStdVectorEigenSpecificType to std-vector.hpp header