Add ATLAS classes needed for CMS+ATLAS combinations

[lviliani]

Including two ATLAS custom classes: RooTwoSidedCBShape and ResponseFunction.
This is basically a copy & paste of the original classes shared by ATLAS, with a couple of minor fixes to avoid compilation errors.

The diffs of the changes are the following.
In AtlasPdfs.cxx:

218c217
<   for (size_t i=0; i < _paramList.getSize(); i++) {
---
>   for (Int_t i=0; i < _paramList.getSize(); i++) {
284c283
<   for (size_t i = 0; i < _paramList.getSize(); i++)
---
>   for (Int_t i = 0; i < _paramList.getSize(); i++)

In AtlasPdf.h:

50d38
<   std::vector<double> _nominal;
51a40
>   std::vector<double> _nominal;
<   
73c61,62
<   ClassDef(ResponseFunction, 1) // flexible interpolation
---
> 
>   ClassDef(ResponseFunction, 2) // flexible interpolation