add a dummy file in every function type directory to document the group

[spinkney]

Right now a random file in each function type directory contains the group definition. It is better to move this into a separate file.

For eg, the copula definition is in clayton_copula.stan as

/** \defgroup copula Copula Functions
  *
  * A copula is a multivariate cumulative distribution function for which the 
  * marginal probability distribution of each variable is uniform on the interval \f$[0, 1]\f$.
  * Copulas are used to describe the dependence between random variables.
  *
  **/

move that out to a file named copula_definition.stan