make objective function simpler

[clarkliming]

currently in mmrm.cpp, in objective function, we have some std::map<int, lower_chol_base<Type>*> object creation, and some ugly clean up in the end.

we should create a class to hold these information to make the objective function cleaner (no new /delete in objective function but inside the class object)

[danielinteractive]

That is a very good idea! And we can test the class etc.