noise.sdf: add min="0.0" to the precision and dynamic_bias_correlation_time parameters

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/231)) by Steve Peters (Bitbucket: [Steven Peters](https://bitbucket.org/%7B2ccfed09-18b8-4921-8d58-15ef01092802%7D/), GitHub: [scpeters](https://github.com/scpeters)).**

----------------------------------------
This is a follow-up task from [pr #329](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/329), which proposed to "Add random walk params to noise for sdf 1.6". Most of the changes in this PR have already been merged in [PR #525](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/525) except for the differences noted in the following comment:

* https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/329/add-random-walk-params-to-noise-for-sdf-16/diff#comment-134273497

Two of the differences are still worth applying, but I'm putting them in an issue to be resolved after the migration to GitHub.

* noise.sdf: add min="0.0" to the precision parameter
* noise.sdf: add min="0.0" to the dynamic_bias_correlation_time parameter