Make diagnostic aggregator composable

[nnarain]

Heya.

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

Summary of changes:

• Have the Aggregator class implement get_node_base_interface so it can be registered as a component
• 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.
  • Consequently this means the top level analyzer group is loaded as a plugin and therefore requires a class loader
• Added compose example launch file.

[frozenreboot]

Hi @nnarain,

Is this PR still being worked on?
If you are busy, I’d be happy to pick it up, resolve the merge conflicts, and submit a new PR referencing your work.
Please let me know!

[nnarain]

hey @frozenreboot

Ya I should be able to get back to this. If I don't find the time this week I'll give you a ping.

Hey @ct2034 any initial feedback on this PR?

[ct2034]

Thanks. This looks very good.
Could I please ask you to provide a small launch test. It could be based in the nice example diagnostic_aggregator/example/compose_example.launch.py.in.
And please also add a small section on this to the diagnostic_aggregator/README.md

[nnarain]

Will do

[nnarain]

There are two failing tests. But it looks like they are failing on the ros2 branch as well.

[mergify]

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