Add functionality to compute function on posterior

Currently, it is very hard to compute a user defined function on the posterior draws. A user asked me if we could add a functionality where the user specifies a function name which is executed on each posterior draw when we compute posterior moments. For such a function call, we would need to clearly specify the interface. We might want to provide M_, oo_, bayestopt_, estim_params_ and the current parameter draw as generic inputs (and nothing else). The tricky part consists of specifying the output arguments and making sure we can save them. One way would be to only accept one output argument and save it to the ii element of a cell array. By putting multiple outputs into a structure, the user could still effectively save multiple outputs while keeping a clearly specified interface for us. Upon going over all draws, we would just save the cell array to the disk. It would be up to the user to make sense of the saved stuff.