Print aux separately

[jgabry]

Separates the printing of auxiliary variables (and marks them as such) in order to distinguish them from regression coefficients in the printed output.

stan_glm
 family:       gaussian [identity]
 formula:      mpg ~ wt + cyl
 observations: 32
 predictors:   3
------
            Median MAD_SD
(Intercept) 39.7    1.7  
wt          -3.2    0.7  
cyl         -1.5    0.4  

Auxiliary parameter(s):
      Median MAD_SD
sigma 2.6    0.4   

This should be clear to the user since sigma (and the other auxiliary parameters for other GLMs) are specified via prior_aux and are referred to as auxiliary parameters in the doc.

Closes #284

[jgabry]

The PR making mean_PPD optional will require this so merging this PR first