spec: Should fix ambiguities for //inertial/{pose,mass,inertia}

[EricCousineau-TRI]

Ambiguities raised by @mitiguy in email thread. Relates #18

Items:

• Update SDFormat specification
• Update URDF XML specification analogously: http://wiki.ros.org/urdf/XML/link
• Fix website rendering for links

[EricCousineau-TRI]

Made changes to URDF as seen here: https://wiki.ros.org/action/diff/urdf/XML/link?action=diff&rev1=76&rev2=77

[scpeters]

Made changes to URDF as seen here: https://wiki.ros.org/action/diff/urdf/XML/link?action=diff&rev1=76&rev2=77

I made a minor to fix to one of your added sentences: https://wiki.ros.org/action/info/urdf/XML/link?action=diff&rev2=78&rev1=77

otherwise it looks good

[scpeters]

draft pull request to fix documentation source for sdformat.org

[scpeters]

I have merged gazebosim/sdformat#990 and uploaded the revised specification files

http://sdformat.org/spec?ver=1.9&elem=link#link_inertial

I believe the only remaining action item is ensuring that it renders properly on the website

[EricCousineau-TRI]

Deferring website rendering to internal web app issue 51

[mitiguy]

Thanks Eric and all. Nice work. Much appreciated.