api cleanup: use ByTag consistently (emphasises slowness of lookup)

* revert last commit from laurent
* new getNumElements() et getElement() functions in GEdge/Face/Region to
  simplify access to mesh

getTypeOfElements  and addThisTypeOfElement
to GEntities

added
getTypeOfElements  from Gentities and Physical Groups in GModel

fix gcc warnings

- clean up GModel API (xxByTag -> getXx, numXx -> getNumXxes, ...)

make it easier to see when char lengths are bad

ooops!

Smoother Surface{list} = numsteps;

removed unused fct

removed GFace::GraphicsRep and reimplemented it using the more general
STL vertex array