New function tixiRemoveExternalLinks

[rainman110]

Tixi has a great feature that allow embedding external xml nodes into another xml element.

To allow writing into the same file structure, tixi needs to store metadata about the file locations, paths and filenames.

These metadata are stored as XML attributes into the document. These added attributes however make it very hard to validate the xml file against an xml schema.

Therefore, we need a function that allows to remove these metadata. I propose "tixiRemoveExternalLinks".