Upgrade alert when petsc updates to python3

[burgreen]

When PETsc upgrades to Python3 compatibility in early 2019, at least one small change needs to be made to DaetkPetscVec.cpp:

replace VecScatterCreate with VecScatterCreateWithData

[jedbrown]

You don't have to leave this hanging (and it's unrelated from Python3 configure except that PETSc 3.11 will have both changes). You can upgrade the call sites now and add this so it continues to work with the current release.

#if PETSC_VERSION_LT(3,11,0)
#  define VecScatterCreateWithData VecScatterCreate
#endif