Figure out how to build FrameAttachedToGraph for static models with no links

[scpeters]

In the initial version of the frame semantics proposal, all models were required to have at least one link, so that they could have a canonical link to which the implicit model frame (__model__) would be attached. This requirement was relaxed in the proposal in Bitbucket sdf_tutorials PR 45 (gazebosim/sdf_tutorials@dea8c40) so that static models would not be required to have a link with the rationale that its frames could be attached directly to the world. This was manifested in libsdformat with Bitbucket sdformat pull request 626 (4fa3f81?w=1), which stopped reporting errors for static models without a single link. Eliminating that error was the primary goal of that PR, and rather than also modify buildFrameAttachedToGraph to handle static models without links in that PR, it just skips building the FrameAttachedToGraph for all static models regardless of the number of links they have.

So, figure out how to build FrameAttachedToGraph for static models with no links.

[scpeters]

See also some initial thoughts here: https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/626/page/1#comment-127736189

[azeey]

Can this be closed since #381 builds the FrameAttachedToGraph for static models?

[scpeters]

sounds reasonable to me