Add gz sdf command that converts a file to an older version of the spec

[scpeters]

Desired behavior

If I have a file using new features of the SDFormat spec, it means that it can't be loaded in software that only supports older versions of the spec (such as gzweb, which only supports up to SDFormat 1.6). It would be helpful to have a tool that could convert a file with version 1.7 or newer back to 1.6.

Alternatives considered

Implementation suggestion

Provide the interface as a --convert-to-version option to gz sdf --print:

gz sdf --print <new_file.sdf> --convert-to-version <old_version>

A short list of the types of conversions to be expected when converting between each version:

• 1.7 -> 1.6: resolve all poses with the default value of //pose/@relative_to
• 1.8 -> 1.7:
  • resolve joint parent/child frames to a link name or world
  • error if ellipsoid geometry is detected, or replace it with an equivalently scaled mesh
  • replace capsule geometries with equivalent cylinder and spheres
• 1.9 -> 1.8: convert all poses to use Euler angles in radians
• 1.10 -> 1.9: error if //world/joint tags are detected
• 1.11 -> 1.10: resolve all auto-inertial values and print in an <inertial> block

It may be simplest to start with a tool that converts SDFormat 1.11 files to 1.10 by expanding any auto-inertial blocks to contain the full computed value.

Additional context