Consider renaming Joint::ParentLinkName and Joint::ChildLinkName to Joint::ParentName and Joint::ChildName

[azeey]

Desired behavior

As of SDFormat 1.8, joints can have frames as parent or child elements, so the functions Joint::ParentLinkName and Joint::ChildLinkName are no longer accurate. My suggestion would be to rename them to Joint::ParentName and Joint::ChildName.

Alternatives considered

They could also be named Joint::ParentFrameName and Joint::ChildFrameName since all parent/child elements are frames, either explicit or implicit. However, I think this is a little more confusing if users are not familiar with the explicit/implicit frame concept.

[scpeters]

see #1053