Extend eigen-accel conversions (backport #887)

[mergify]

As promised in my previous PR, I have extended the conversions by introducing the toMsg for converting an Eigen matrix to an Accel message. I kept the function signature similar to other toMsg ones that have ambiguity with other types (e.g., Vector3d and Point or Vector3 and Point32).

Furthuremore, I added some tests for both fromMsg as well as toMsg functions of eigen-accel conversions.

The only thing that I am a bit doubting is why in docstrings of such toMsg functions it is always said:
This function is a specialization of the toMsg template defined in tf2/convert.h.
I could not find any toMsg template with such function signature i.e., two inputs. There's only this:

template<typename A, typename B>
B toMsg(const A & a);

Am I missing something? In any case, I have kept the docstring the same also here.

---

This is an automatic backport of pull request #887 done by Mergify.

[ahcorde]

Pulls: #891
Gist: https://gist.githubusercontent.com/ahcorde/b6204d9a5bbea4b20d1ce1f6af08e4e3/raw/ba17582fecd69a622671d273d985f8879d33acbf/ros2.repos
BUILD args: --packages-above-and-dependencies tf2_eigen test_tf2
TEST args: --packages-above tf2_eigen test_tf2
ROS Distro: jazzy
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/17883

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