add gmx namespace declaration

[tomkralidis]

Fixes missing XML namespace declatation binding in ISO metadata.

Checklist

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[cla-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @tomkralidis on file. In order for us to review and merge your code, please contact the project maintainers to get yourself added.

[tomkralidis]

Ah, looks like @afabiani fixed this at the same time :) a0f09cd

Closing -- thanks !