XML attribute normalization is missing

[bubnikv]

lib3mf could read the XML attribute values it writes, however it seems to not implement XML attribute normalization:
https://www.w3.org/TR/REC-xml/#AVNormalize

Namely, if a XML attribute value contains newlines or tabs, these newlines or tabs are replaced by libexpat by spaces after parsing and before passing to an application due to the mandatory normalization as shown by the following table from the XML norm:

Please note that an attribute type is considered CDATA by default.

[bubnikv]

And by the way, none of the XML parsers / writers that I checked (boost property tree, boost serialization, tinyxml) seem to implement normalization.

Also if a producer escapes newlines with &#xD, it seems that lib3mf will NOT unescape such sequence.