[RF] Use input argument version of getObservables in RooAbsPdf
[RF] Use input argument version of getObservables in RooAbsPdf

This avoids some RooArgSet creations on the heap that happened quite
frequently in RooAbsPdf (to the point that it was noticable in
benchmarks).

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root