Make diagnostic aggregator composable

[nnarain]

Heya.

This PR makes the diagnostic_aggregator node composable (see #321 )

Summary of changes:

• Have the Aggregator class inherit from rclcpp::Node. Removing the std::shared_ptr<Node> member variable.
• Change the Analyzer base class to accept a Node& instead of the std::shared_ptr
• Register the aggregator as a rclcpp_component
• analyzer_group.cpp is moved to the analyzers library
  • The plugins xml file incorrectly lists the AnalyzerGroup plugin as existing the analyzers library. When composing the nodes I found that AnalyzerGroup would not load. I assume it has worked up to this point because everything was linked into the same executable.
• Added compose example launch file.