Reorganize build to separate core functionality from future ROS2 development

[jjd9]

As discussed here https://github.com/facontidavide/Bonxai/pull/23, this PR is the first of a small sequence of PR's intended to add rviz2 support for Bonxai. This PR reorganizes the repo slightly to make it easier to add more ROS packages (E.g. msg/srv definitions, rviz plugin, etc...) to this repo without confusing usage for people who do not want to use the ROS2 parts of the repo.

Core functionality is moved to Bonxai subdirectory ROS aspects are moved into Bonxai_ros subdirectory

The intended build approach is explained in the updates to the README.

This organization made sense to me, but I am happy to make changes if anyone has other ideas.

Thanks for your time! (-: