[ament_cppcheck] add --enable=style

[vincentrou]

I saw that coding style checks of cppcheck is not enabled : https://github.com/ament/ament_lint/blob/rolling/ament_cppcheck/ament_cppcheck/main.py#L145
For example, unused variables can be checked with this option.
Could you consider to add it ? I can make a PR for this.

[clalancette]

Honestly, cppcheck is more-or-less completely disabled at the moment: https://github.com/ament/ament_lint/blob/rolling/ament_cppcheck/ament_cppcheck/main.py#L127 . It's just way too slow to use (cppcheck 2.x is something like 10x slower than cppcheck 1.9).

Additionally, even if we re-enable it, we already have 2 style checkers enabled for ROS 2 by default; uncrustify and cpplint. Adding a third one would most likely make it hard to please all of them.

That said, if you'd like to add a way to optionally enable style using cppcheck, please feel free to open a PR. We'll be happy to review.

[vincentrou]

@clalancette Is it possible to review the PR #407 ?