Adapt code to use the intended (list of) StreamerElement

This is ‘instead’ of re-compiling the StreamerInfo to force the optimized
list to be of the type (optimized/non-optimized) the calling code needed.

Add to TVirtualStreamerInfo the routines:

    TStreamerElement *GetElement(Int_t id) const;
    Int_t  GetElementOffset(Int_t id) const;

respectively as a convenience accessor and as a performance accessor
to the StreamerElement information based on ‘id’ being the index
in the full list of StreamerElement (in contrast GetElem and GetOffset
take an id which is the index in the shorter list of optimized elements).

Add TStreamerInfo::GetNelement() as a short cut to  fElements->GetEntries();

Conflicts:
	io/io/src/TStreamerInfoActions.cxx