to fix on 18.04:
```
[ 94%] Building CXX object unittest/CMakeFiles/std_pair.dir/std_pair.cpp.o
cd /local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/unittest && /usr/bin/g++  -Dstd_pair_EXPORTS -I/local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0 -isystem /local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/include -isystem /usr/lib/python3/dist-packages/numpy/core/include -isystem /usr/include/python3.6 -isystem /usr/include/eigen3  -Wno-long-long -Wall -Wextra -Wcast-align -Wcast-qual -Wformat -Wwrite-strings -Wconversion   -pipe  -O3 -DNDEBUG -fPIC   -std=gnu++14 -o CMakeFiles/std_pair.dir/std_pair.cpp.o -c /local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/unittest/std_pair.cpp
/local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/unittest/std_pair.cpp: In function 'void init_module_std_pair()':
/local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/unittest/std_pair.cpp:28:3: error: reference to 'bp' is ambiguous
bp::def("std_pair_to_tuple", std_pair_to_tuple);
^~
/local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/unittest/std_pair.cpp:8:29: note: candidates are: namespace bp = boost::boost::python;
namespace bp = boost::python;
^
In file included from /local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/include/eigenpy/eigenpy.hpp:9:0,
from /local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/unittest/std_pair.cpp:4:
/local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/include/eigenpy/fwd.hpp:81:29: note:                 namespace eigenpy::bp = boost::boost::python;
namespace bp = boost::python;
^
/local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/unittest/std_pair.cpp:29:3: error: reference to 'bp' is ambiguous
bp::def("copy", copy);
^~
/local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/unittest/std_pair.cpp:8:29: note: candidates are: namespace bp = boost::boost::python;
namespace bp = boost::python;
^
In file included from /local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/include/eigenpy/eigenpy.hpp:9:0,
from /local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/unittest/std_pair.cpp:4:
/local/robotpkg/var/tmp/robotpkg/math/py-eigenpy/work/eigenpy-3.3.0/include/eigenpy/fwd.hpp:81:29: note:                 namespace eigenpy::bp = boost::boost::python;
namespace bp = boost::python;
^
```

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