IDL4CPP — std::shared_ptr should be passed by value

The spec in 7.4 says that only basic types/enum should be passed by value for an in argument, but that should also include all shared_ptr types, see https://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/ for some information (more can be found on the internet).

Passing shared_ptr by value could introduce certain issues

If we pass by value we could introduce certain issues, like locking (having to increase the reference counter). It's unclear to us whether this would be beneficial. If we pass it by const reference, we're guaranteeing that the object won't be destroyed until the end of the function.

After a discussion within the IDL WG, we agreed on leaving the mapping as it currently is and reevaluate improvements to the mapping in future RTFs.