Small changes to Mesh

o Moving GRID_LOAD macros from mesh.hxx to globals.hxx, since they refer
  to the global mesh pointer

o Making GlobalX, XGLOBAL etc. functions const
FieldFactory takes Mesh argument

o Constructor must now be passed a pointer to a Mesh

o Allows FieldFactory to be used for multiple meshes
  (eventual goal of e.g multigrid methods)

o Will allow meshes to be constructed by using
  analytic formulae in BOUT.inp rather than just
  constants at present