[BioPython] KDTree optional compilation

[K. Arun]

Hello,

I just updated to version 1.42 (thanks !), and noticed I had to
uncomment the KDTree entry in setup.py in the NUMPY_PACKAGES section
to use Bio.PDB.NeighborSearch.  Since KDTree is essential to this
module, would it possible to add a note about this in the README, or
print a message during compilation to the effect that setup.py needs
to be modified ?

-arun