Add validation to XML import

[mbollmann]

Originally reported by: Marcel Bollmann (Bitbucket: mbollmann, GitHub: mbollmann)

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We now have a RelaxNG schema and a Python script for validation of CorA XML files. The import of XML files into CorA should probably make use of this to validate files before attempting an import.

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• Bitbucket: https://bitbucket.org/mbollmann/cora/issue/82