Proposal: Make <joint>::<axis>::<limit>::<lower>/<upper> elements optional and default to unbounded

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/169)) by Michael Grey (Bitbucket: [mxgrey](https://bitbucket.org/%7Bc1cdfe52-2887-474c-ae99-72fdc53a59c9%7D/), GitHub: [mxgrey](https://github.com/mxgrey)).**

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According to [the documentation](http://sdformat.org/spec?ver=1.6&elem=joint#limit_lower), if an `` element is specified, then a `` element is required, which then requires both `` and `` elements.

Many users might not realize that they can safely use `inf` or `-inf` in their sdf to express unbounded limits, and that may influence them to exercise questionable practices, like using `99999999999` to get the behavior they want.

We could make `` and `` optional elements whose values default to be unbounded (-/+inf). I think it's intuitive that if a user declines to specify a limit, then we should assume there is no limit.

I believe this change would also be backwards compatible with existing SDFs.

No implementation files, tests, or entry points are named. Begin by locating the parser and schema handling for ::::::/, then verify that omitted bounds default to -/+inf while existing SDFs remain compatible.

Written by the indexing model from the issue text.