Investigate changing branch to line code coverage

[SteveMacenski]

The OSRF Quality level 1 target for coverage of 90% is line coverage not branch coverage.

Currently, our test coverage is branch coverage, which is why it looks very low (well, its definitely not 90% either). Many functions in the C++ standard library can throw exceptions when used incorrectly. Those exceptions stop lines which are functionally covered in all branches of the code to show up as only partially covered because we would have to trigger every single exception possible for every single line (dozens if not hundreds).

We have main() functions with no branching statements with < 30% coverage because rclcpp::init can throw exceptions, vector push_back can throw exceptions, even string additions.

I'd love a PR to change branch coverage to line coverage to analyze the results and help make a final call about which we should be tracking.

[SteveMacenski]

Investigated and discussion in details in #1713