Rename CanonicalLink to RootLink

[azeey]

🦟 Bug fix

Fixes #233

Summary

See #233 for context.
This deprecates the end-user facing function CanonicalLink adds a new one called UnconstrainedLink.

Note: While the changes in this PR won't break downstream users of ign-physics, it will break any physics plugins the implement the FreeGroup feature.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• 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

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[adlarkin]

Changes look OK to me. I'll let @scpeters do one last final pass since I'm still pretty new to ign-physics.

Should we add a note to the migration guide about this change so that other users who have a plugin that implements the FreeGroup feature know what updates to make to reflect this change?

[scpeters]

looks good, just need a migration guide entry

[mxgrey]

I might make a suggestion about the naming:

UnconstrainedLink sounds like it refers to a link that does not have any constraints attached to it. I.e. it has no parent nor child joints.

If Canonical is not considered a suitable name anymore, then instead of UnconstrainedLink I might recommend calling this the RootLink. If it's called RootLink then that explicitly conveys that it doesn't have parent joints but may have child joints.

[azeey]

I might make a suggestion about the naming:

UnconstrainedLink sounds like it refers to a link that does not have any constraints attached to it. I.e. it has no parent nor child joints.

If Canonical is not considered a suitable name anymore, then instead of UnconstrainedLink I might recommend calling this the RootLink. If it's called RootLink then that explicitly conveys that it doesn't have parent joints but may have child joints.

RootLink make more sense. Renamed in 14a6b21

[azeey]

Ubuntu CI failed due to infra issues.
@osrf-jenkins run tests please