updates to reflect changes in rclcpp, added ros1_like examples
adding WIP advanced explicit examples
modify examples and disable some
remove groups_no_node example, no longer valid
fixup pub/sub examples
different_groups: remove vestigial code
fixup talker example to match changes in rclcpp
[style] fixup whitespace in CMakeLists.txt
make sure examples use rclcpp::init
Merge pull request #1 from ros2/executors

Adding examples which use executors