Fixed XML declaration handling in FunctionExternalCommand
* `FunctionExternalCommand` failed to offer correct output handling when provided with a valid XML document which included a XML declaration
** `xerces::XercesDOMParser` generated the appropriate errors but they were not handled by `FunctionExternalCommand`
** wrapping is not necessary for full XML documents but is required for invalid XML as returned by e.g. common _Markdown_ processors
*** if wrapping is applied to full XML documents which include a XML declaration the parser and subsequently the complete external function fails
* added error parser handling by making the compilation unit local `importDocumentElement` method return value optional
* added XML declaration detection and handling to `readOutput`
** if XML declaration is detected no wrapping is applied
* added wrapping detection in `FunctionExternalCommand::constructDocument`
** if no wrapping is detected the whole tree is returned as content instead of just returning its child nodes