Support custom sensors

[chapulina]

🎉 New feature

Closes #9

Builds on top of #90

Summary

Add support for custom sensors that aren't installed together with ign-sensors, so that external users can implement their own sensors. This adds:

• The environment variable IGN_SENSORS_PATH that users can set to find sensors.
• An example with a custom sensor
• An example with an executable that can loop any non-rendering sensor, and can be used to test the custom sensor

It also makes the deprecated Update function non-pure-virtual, so downstream developers aren't forced to implement a deprecated function.

I decided to work on this to validate that the work being done on #90 was flexible enough to support external sensors.

Test it

Follow the instructions on each example's READMEs.

Checklist

• Signed all commits for DCO
• Added tests ⚠️ TODO
• Added example and/or tutorial ⚠️ TODO I want to move the READMEs to tutorials
• Updated documentation (as needed) ⚠️ TODO document IGN_SENSORS_PATH somewhere
• Updated migration guide (as needed)
• 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

[peci1]

Great you started working on this. So how does this go together with the SDF spec? In #9, you mentioned custom namespaces, but that's not the case used in the example. What if I needed a some XML parameters completely unknown to the SDF spec?

Also, the wording of http://sdformat.org/spec?ver=1.7&elem=sensor#sensor_type could be changed to better reflect the fact that plugins can add more supported sensor types.

[chapulina]

Removing beta label, we won't have time to wrap this up before code freeze. Let's retarget at Ignition-F.