Create node executables using rclcpp_component macros

[bjsowa]

rclcpp_components_register_node macro can create templated source for node executable that runs the Node component.

This PR replaces some boilerplate code using this macro.

[jonbinney]

I like using the macros, but I think there's still a good reason to put them in separate _node source files. In #188 the main function was split into a separate file so that the filter chain component classes could be #included by other people's code without getting a main function.

[bjsowa]

I like using the macros, but I think there's still a good reason to put them in separate _node source files.

Sure, If you feel so, you can close this PR. The macros just seem "cleaner" to me but it's just my opinion.

If you want to stick to _node source file, I will at least try to get 0597364 merged.

[jonbinney]

I read up on rclcpp_components_register_node and now this PR makes sense to me. Once you add the suggested comment, I'll merge this.

[jonbinney]

Could you add a comment describing what rclcpp_components_register_node does here? It's a very cool and useful macro, but the documentation isn't super easy to find, and it'd be nice to have a summary here.

[bjsowa]

I added a comment that briefly describes the macro

[jonbinney]

Looks good, merging! btw i find it very odd that rclcpp_components_register_node creates the templated node executable, but rclcpp_components_register_nodes (plural) doesn't.....