[This is preliminary documentation and is subject to change.]
Get the SIF namespace for this version of the specification.
If the SifVersion is less than SIF 1.1, a namespace in the form
"http://www.sifinfo.org/v1.0r2/messages" is returned, where the full
SIF Version number is included in the namespace. For SIF 1.x and
later, a namespace in the form "http://www.sifinfo.org/infrastructure/1.x"
is returned, where only the major version number is included in the
namespace.