Joint type "continuous" and "revolute" appear redundant

SDFormat 1.6/1.7/1.8 XML has both a `revolute` and a `continuous` joint type, but they appear to be redundant. A `continuous` joint has no limits, while a revolute joint does. However, whether or not a joint has a limit is already given by the presence of a `` tag in the `` on the joint. Does `type="continuous"` mean a `` on the `` is an error? Similarly does `type="revolute"` mean the lack of an `` with a `` is an error?

It would seem that the `continuous` joint type could be removed if the spec was changed to say a joint of type `revolute` is limited if it has a ``, otherwise it's continuous.

https://github.com/osrf/sdformat/blob/f94626a8898b0a2898ae65892d76fd1031456133/sdf/1.7/joint.sdf#L11-L12