Add multirobot example

[gbiggs]

This PR adds a multirobot example using Drake-ROS. It creates a two-dimensional array of Kuka LBR iiwa robots, which can then be visualised using rviz.

See the README.md file for how to run the example.

Questions that need answering:

• Should the iiwa_description directory be made its own package? (Or, is it already a package somewhere that we can use?)
• Should we add a C++ version of the sample as well?
• Is "the robots all fall down due to gravity" a sufficient example, or should the robots do something?

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This change is 

[EricCousineau-TRI]

Reviewable status: 0 of 62 files reviewed, 2 unresolved discussions (waiting on @EricCousineau-TRI and @gbiggs)

a discussion (no related file):

Should the iiwa_description directory be made its own package? (Or, is it already a package somewhere that we can use?)

We should use Drake's version, and not re-vendor it here.

And I believe our version does not use /robot_description, so we need not expose it (yet) on the package path.

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a discussion (no related file):

Should we add a C++ version of the sample as well?

Nah, I think Python is sufficient!

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a discussion (no related file):

Is "the robots all fall down due to gravity" a sufficient example, or should the robots do something?

I think it's fine as long as it's clearly indicated somewhere (README) that this is the expected behavior, e.g.

This will simulate the array of IIWA robots without control. The expected behavior is for
them to "flop" about as simulation time progresses.

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[gbiggs]

@IanTheEngineer Do you have a suggestion on how to find the location of the iiwa_description included with Drake? The virtual environment directory could be anywhere.

My first thought is to configure a Python file using CMake so that it contains the path as found by CMake's find_package(drake), but the downside of that is that it is fixed. If the location of the virtual environment containing drake changes, then it will break.

[EricCousineau-TRI]

Depends on #39

[EricCousineau-TRI]

From Anzu:

PackageMap MakeDefaultAnzuPackageMap() {
  PackageMap package_map;

  // Add ability to read `package://anzu` URIs.
  const anzu::filesystem::path packagefile_abspath =
      FindAnzuResourceOrThrow("package.xml");
  package_map.Add("anzu", packagefile_abspath.parent_path());

  // Add packages from drake.
  const anzu::filesystem::path abspath_under_drake_root =
      drake::FindResourceOrThrow("drake/.drake-find_resource-sentinel");
  package_map.PopulateFromFolder(abspath_under_drake_root.parent_path());

  return package_map;
}

[jwnimmer-tri]

// Add packages from drake.

That stanza is no longer relevant (it used to be relevant last year). The drake::multibody::PackageMap by default has the (sole) Drake package already added. There is no need to add any additional packages from Drake.

[EricCousineau-TRI]

ah, kewl. will scrub that from Anzu's code then - thanks!

[gbiggs]

Thanks for the tip, @EricCousineau-TRI . I've got it working.

However, somewhere in my rebasing I messed up, somehow, and now this PR wants to remove all of the develop branch's content. I've given up trying to clean it up and just made a new branch and PR over at #85.