0020978: EDF 1475 SMESH: Convert linear to quadratic on a submesh

0020978: EDF 1475 SMESH: Convert linear to quadratic on a submesh
  fix the problem with adding elements (elements are not added) after
  {elems removal + addition of elements with specified IDs}:
  1) add elems with IDs 1,2,3
  2) remove elem 1 (1 stored in the pool )
  3) add elem with id 1 (1 remains in the pool)
  4) add elem with no id specified -> not added as GetFreeID() returns
     1 which is already occupied

+  virtual int GetFreeID();