Returns the priority level of the marshal. Marshals with higher priority levels are always
invoked by the
MarshalManager prior to marshals with lower priority levels.
The recommendation is that the priority level should increase when the content is
identifiable through an increase in specificity for the data stream. For example, a
Marshal that detects a stream as XML should return a priority level of
0. A Marshal that detects CoT should return a level of
1 and a Marshal that detects KML should return a level of
1 as both are direct derivatives of XML. A Marshal that detects a
specific type of CoT should return a level of 2 to ensure it gets invoked prior
to a generic CoT Marshal.