Add HasConnections function

[iche033]

Signed-off-by: Ian Chen [email protected]

🎉 New feature

Summary

Add HasConnection function to all sensors. The main motivation is to so that users of sensors can check to see if there are any subscribers / connections to the sensor data. If none, they can avoid doing any unnecessary computation, see gazebosim/gz-sim#1480

Each sensor has its own publishers and events so each needs different implementation. Note that I had to change the GpuLidarSensor class' event callback implementation slightly so that I can keep track of the event connection count. The changes also make it more consistent with other rendering sensors event callbacks.

Test it

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #222 (547505d) into ign-sensors6 (debf56e) will not change coverage.
The diff coverage is n/a.

❗ Current head 547505d differs from pull request most recent head bc0dfca. Consider uploading reports for the commit bc0dfca to get more accurate results

@@              Coverage Diff              @@
##           ign-sensors6     #222   +/-   ##
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  Coverage         80.00%   80.00%           
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  Files                 1        1           
  Lines                15       15           
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  Hits                 12       12           
  Misses                3        3           

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