Mark optional dependencies in package.xml.

[brianhou]

Depends on personalrobotics/pr-cleanroom#14.

In the CI scripts, we should now run something like

./scripts/internal-distro.py ... -o OPTIONAL

where OPTIONAL is the name of the dependency group below.

---

Before creating a pull request

• Document new methods and classes
• Format code with make format

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[jslee02]

Looks good to me.

Once this PR is merged, I'll update #373 reflecting this change and personalrobotics/pr-cleanroom#14.

[codecov]

Codecov Report

Merging #376 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
- Coverage   83.48%   83.46%   -0.02%     
==========================================
  Files         197      197              
  Lines        5619     5619              
==========================================
- Hits         4691     4690       -1     
- Misses        928      929       +1

Impacted Files	Coverage Δ
src/planner/ompl/CRRT.cpp	75.58% <0%> (-0.59%)	⬇️

[gilwoolee]

Can we update the readme and say that these needs to be uncommented for using some of of the underlying packages?

[aditya-vk]

Looks like libherb and herb3_demos won't pass any CI tests until the optional dependencies issue isn't fixed. We should probably close this with a neat solution [force all optional dependencies to be installed?] soon before merging any PRs in the other downstream packages?

[brianhou]

Here's another proposal for managing optional dependencies:

• By default, all dependencies (including optional dependencies) are required.
• Introduce <optional> tags (inside <export>) to mark which of the <depend>s are optional. We should be able to programmatically access those in the pr-cleanroom/internal-distro.py script and remove them from the set of dependencies (based on some CLI flag). We'll only do this when testing on Travis.

I've updated package.xml to match this proposal, and will update the internal-distro.py script next.

Edit: depends on personalrobotics/pr-cleanroom#16.