- all functions now return a vector<int>
- first entry is the return code
- any integer output value is appended

This makes it as natural to use when specifying tags explicitly or when letting Gmsh return new ones

- refine GetEntitiesInBoundingBox, GetMeshSize and SetTransfinite API
- refine

- renamed old "Gmsh.h" -> "GmshGlobal.h"
- use lowercase for library name

- removed onelab::region and onelab::function (they complexify the interface, but bring no additional value compared to onelab::string)
- removed ONELAB2 experiment
- made client/Model name persistent, and define Gmsh/Model name so that we can uncouple e.g. .geo/.pro pairs

Keep essential objects for multiple calls to gmsh lib. In particular, do not delete PViewOptions::reference() and CTX::instance().