Builds a message based on a XML file (the template), substituting the vales
in the file with the values specied in the paramaters.
The syntax of a parameters are:
[XPath expression that selects the single attribute or element ]==[value that
should be assigned to the field] If the namespace prefixes are used in
the template file, the same prefixes must be used in the XPath expression.
If the a default namespace is used in the template file, 'def' must by used a
namespace prefix in the XPath expression
