Add round-trip conversion tests and verify same number of entities before and after

[scpeters]

Desired behavior

Several recent bugs have noted that some entities that are properly converted by sdf2usd are not properly converted back by usd2sdf:

• usd2sdf: capsule and ellipsoid geometries are not exported #41
• usd2sdf: visual elements are not exported #42

The reproduction instructions in those issues describe converting an SDFormat file to USD using sdf2usd and then back again to SDFormat using usd2sdf. Without expecting that the SDFormat files before and after the round-trip conversion should be identical, we should expect the same number of entities (links, collisions, visuals, shape geometries, joints, etc) in both SDFormat files. It would be useful to have testing infrastructure to enable this type of round-trip testing to enable entity counts.

Alternatives considered

Implementation suggestion

Additional context