Add wiener distribution

Code from by Joachim Vandekerckhove http://www.cidlab.com/supp.php?o=wienerstan, which was created for stan v1.3; tweaked by Mike Lawrence to (hopefully) work for 2.1.0.
That's curious --- we don't have a laplace function
implemented.  We have the double exponential distribution, which
is sometimes called the laplace distribution.  But we don't doc it.

I think it should just be removed.

- Bob


On Jun 26, 2014, at 11:37 PM, Peter Li <notifications@github.com> wrote:

> In src/stan/gm/function_signatures.h:
> 
> > @@ -371,6 +371,13 @@ add("inv_wishart_log",DOUBLE_T, MATRIX_T,DOUBLE_T,MATRIX_T);
> >  add("inv_wishart_rng",MATRIX_T,DOUBLE_T,MATRIX_T);
> >  add("inverse",MATRIX_T,MATRIX_T);
> >  add("inverse_spd",MATRIX_T,MATRIX_T);
> > +for (size_t i = 0; i < vector_types.size(); ++i)
> > +  for (size_t j = 0; j < vector_types.size(); ++j)
> > +      add("laplace_log",
> 
> Why is this laplace?
> 
> —
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