Adds a new attribute with a double value to the current element.
Use null ns_href, to use the default namespace, otherwise either previously added
namespace with the same href from e_xml_document_add_namespaces is picked, or a new namespace with generated prefix is added.
| this |
an XmlDocument |
| ns_href |
optional namespace href for the new attribute, or null |
| name |
name of the attribute |
| value |
double value of the attribute |