RMSE = prtScoreRmse(GUESS, TRUTH)
 
    RMSE = prtScoreRmse(GUESS, TRUTH) returns the
    root mean squared error between the guesses in GUESS as compared to the truth in TRUTH.
    GUESS and TRUTH should both be Nx1 vectors, or both be prtDataSets.  If
    they are prtDataSets, TRUTH.isLabeled must be true.
 
    Example:
    dataSet = prtDataGenNoisySinc;   % Load a prtDataRegress data set, a
                                     % noisy Sinc function
    reg = prtRegressRvm;             % Create a prtRegressRvm object
    reg = reg.train(dataSet);        % Train the prtRegressRvm object
    dataSetOut = reg.run(dataSet);   % Run the regressor on the data
 
    truth = sinc(dataSet.getX);
    guess = dataSetOut.getX;
    prtScoreRmse(truth, guess)