Validates a USI (Unique Student Identifier). 

This is intended to be used during user input (eg. during an online enrolment) to check that a student has entered a valid USI. 

Note that currently this function only checks that the USI is well-formed. It does not communicate with the USI web services to formally verify that the USI is valid. 

A token is passed for authentication. If the authentication fails, or other errors occur, the USI is not validated. The result of the authentication is represented by a TAuthenticate object. 

This function has security level "Unrestricted", and can be accessed by any authenticated user.

[WebMethod(Description = "Validates a USI")]
public TAuthenticate ValidateUSI(string token, string usi);