Should rename the term "Canonical link" in FreeGroup

[azeey]

Desired behavior

The term "canonical link" in SDFormat refers to a link to which the model frame is attached. That is, there's a coordinate frame called the model frame and it has a constant pose offset from the implicit frame defined by the canonical link. On the other hand, the term "canonical link" in the context of FreeGroup refers to a link in the model that has no constraints. For DART, this link would be the root of a kinematic tree and it's parent joint would be FreeJoint, which has 6DOF. Thus it is possible that the "canonical link" of a model per SDFormat be different from the "canonical link" of the FreeGroup of the model

For example:

<?xml version="1.0"?>
<sdf version="1.7">
  <model name="M1">
    <link name="L1"/>
    <link name="L2"/>
    <joint name="J1" type="fixed">
      <parent>L2</parent>
      <child>L1</child>
    </joint>
  </model>
</sdf>

The canonical link of M1 per SDFormat is the link L1, however the canonical link of the FreeGroup of M1 is L2 because L1 and L2 are constrained by a joint and L2 is the parent.

This is also exacerbated by the fact that starting in SDFormat 1.7, the canonical link can be specified by the user to be any link in the model.

Since the term is overloaded, and the Gazebo/SDFormat version of canonical link existed (maybe implicitly) before the FreeGroup counterpart, we should come up with a new name for FreeGroup's canonical link.

Implementation suggestion

Rename the FreeGroup "canonical link" to "unconstrained link"

[scpeters]

there's only a few hours left before feature freeze, but this sounds like an API change to me

[chapulina]

It sounds like a good idea to me. As @scpeters pointed out, we're close to feature freeze though. We could open a pull request starting the implementation. One thing to keep in mind is that the existing public CanonicalLink functions will need to be tick-tocked.

[scpeters]

done in #234

[chapulina]

Just a reminder that ign-gazebo is still using CanonicalLink, which issues a warning. I don't think there's a PR addressing this yet.

[azeey]

gazebosim/gz-sim#723