Updates a single topic score, for a single user.

    Inputs: - host_name: A string containing the address of the machine where the PServer instance is hosted.
            - client_name: The PServer client name.
            - client_pass: The PServer client's password.
            - user_twitter_id: A Twitter user identifier.
            - feature_name: A specific PServer feature name.
            - feature_score: The corresponding score.

def update_feature_value(host_name, client_name, client_pass, user_twitter_id, feature_name, feature_score):
    """
    Updates a single topic score, for a single user.

    Inputs: - host_name: A string containing the address of the machine where the PServer instance is hosted.
            - client_name: The PServer client name.
            - client_pass: The PServer client's password.
            - user_twitter_id: A Twitter user identifier.
            - feature_name: A specific PServer feature name.
            - feature_score: The corresponding score.
    """
    username = str(user_twitter_id)
    feature_value = "{0:.2f}".format(feature_score)
    joined_ftr_value = "ftr_" + feature_name + "=" + str(feature_value)
    values = "usr=%s&%s" % (username, joined_ftr_value)

    # Construct request.
    request = construct_request(model_type="pers",
                                client_name=client_name,
                                client_pass=client_pass,
                                command="setusr",
                                values=values)

    # Send request.
    send_request(host_name,
                 request)