GetUser: Log in using username and password
===========================================

Implement this function:


.. function:: GetUser(username string, password string) (userdataptr *User, err error)

    * Obtains the User struct of a user who has already been initialized and returns a pointer to it.
    * Returns an error if user has not been initialized or user credentials are invalid.
    * Returns an error if the User struct cannot be obtained due to malicious action or the integrity of the User struct has been compromised.

    :param username: Username
    :type username: string
    :param password: Password
    :type password: string

    :rtype: userdataptr, error

.. warning::
    Do not forget that your design must satisfy **all** requirements, including:
    
    #. The client application must be stateless (see
       :ref:`requirement_stateless`).
    
    #. You cannot use global variables to store user information (see
       :ref:`requirement_golang`).

    #. Users can have multiple active user sessions at once (see
       :ref:`requirement_user_sessions`).