Update changelog for 4.1.0

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/117)) by Steve Peters (Bitbucket: [Steven Peters](https://bitbucket.org/%7B2ccfed09-18b8-4921-8d58-15ef01092802%7D/), GitHub: [scpeters](https://github.com/scpeters)).**

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Version 4.1.0 was released and the changelog doesn't reflect all the pull requests:

https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/branches/compare/sdformat4_4.1.0%0Dsdformat4_4.0.0#pull-requests

https://github.com/osrf/sdformat/blob/master/Changelog.md

Review the 4.1.0 versus 4.0.0 pull-request comparison, then inspect Changelog.md. Update the changelog so it reflects all pull requests included in version 4.1.0, and verify that the documented entries match the release comparison.

Written by the indexing model from the issue text.