Should support SDFormat to URDF conversion

Should ensure that SDFormat models are compatible (e.g. no loops, etc.).
URIs are up for debate for how to handle / convert (if at all).

Can help with https://github.com/osrf/sdformat/issues/265, e.g. making sure compatibility checks are good, exposing that as public API.

\cc @IanTheEngineer

Review the compatibility requirements for converting SDFormat models to URDF, including loop detection and the unresolved URI handling question. Start with the compatibility checks discussed in osrf/sdformat#265 and determine how exposing those checks as a public API should be defined. Done means the compatibility scope and URI behavior are decided and the checks/API are implemented.

Written by the indexing model from the issue text.