Add Sensor class

[iche033]

🎉 New feature

Part of #325

Summary

Add generic Sensor class. Currently there are only a small set of API functions due to limited number of sensor components available.

In order to have APIs for doing things like setting update rate or active state, etc, we'll need to create new components and make use of them in all the relevant systems (e.g. imu, altimeter, sensors system, etc).

I also added one more API (ParentModel) to the Joint class that I missed in #1910.

Test it

Run the UNIT_Sensor_TEST and INTEGRATION_sensor tests.

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 #1912 (633d1bd) into ign-gazebo6 (89df3b7) will increase coverage by 0.00%.
The diff coverage is 87.87%.

❗ Current head 633d1bd differs from pull request most recent head 4610842. Consider uploading reports for the commit 4610842 to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo6    #1912   +/-   ##
============================================
  Coverage        64.99%   65.00%           
============================================
  Files              324      325    +1     
  Lines            26562    26595   +33     
============================================
+ Hits             17264    17287   +23     
- Misses            9298     9308   +10     

Impacted Files	Coverage Δ
src/Sensor.cc	85.71% <85.71%> (ø)
src/Joint.cc	97.10% <100.00%> (+0.10%)	⬆️
src/SimulationRunner.cc	90.82% <0.00%> (-0.95%)	⬇️

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[ahcorde]

@osrf-jenkins run tests please!