Update cmake to include new _LIBRARIES fix

Return by ref when possible

Fix destruction of objects 

Sync submodule CMake

Sync submodule CMake

Fix packaging

Remove silly warnings

Authorize the sharing of memory between Eigen and Numpy

Remove eigenpy::Ref

No more useful

[CI] update gitlab CI

from the generated configuration at
https://rainboard.laas.fr/project/eigenpy/.gitlab-ci.yml

This:
- add the doc-coverage target, to save the generated documentation an
  code-coverage results
- add the format target, to check the code formatting (allow_failure is
  activated here for now), ref https://github.com/gepetto/linters
- set 18.04 as the main platform (for now, this affects only format &
  doc-coverage target)
- allow automatic retries in case of runner failures
- drop 14.04 checks