Update the SDFGenerator to use SDF DOM

[nkoenig]

Desired behavior

The SDFGenerator code manually creates SDF elements instead of using the SDF DOM. The problem is that this code can be difficult to maintain, and it's missing some features.

Implementation suggestion

Update the SdfGenerator functions to use the SDF DOM objects. This should make the output consistent with other users of libsdformat, and can take advantage of SDF improvements as they are released without requiring additional updates to the SDFGenerator code.