[feature] parameter to add extra URDFs

[ubgk]

This PR introduces a new parameter (i.e. a vector of strings) that can be used to specify additional URDFs to be loaded into simulation at launch time.

Closes #80.

[coveralls]

Pull Request Test Coverage Report for Build 7726630738

• -4 of 5 (20.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 89.431%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
vulp/actuation/BulletInterface.cpp	1	5	20.0%

Totals
Change from base Build 7608961794:	-0.2%
Covered Lines:	1760
Relevant Lines:	1968

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💛 - Coveralls

[stephane-caron]

LGTM 👍

If you feel so inclined you can also blast loading a ground plane URDF (removing the dependency on bazel::tools::cpp::runfiles::Runfiles) and pass the ground as an extra URDF in the Upkie spine. This would help with @pgraverdy's work where we cannot rely on Bazel runfiles any more.

[ubgk]

Circling back on this, can we just not set params.floor to false and pass the known ground plane URDF in params.extra_urdf_paths?

cc @pgraverdy

[stephane-caron]

Yes, and once that works, you can remove params.floor altogether 😉