[mpiwg-coll] Review implementation of #419+420 in chapter 7 - take 2

[Torsten Hoefler]

Dear Chapter Committee (and anybody interested),

Thanks to all for your review! Puri found some formatting 
inconsistencies (only missing spaces and line breaks) that took a while 
to resolve (thanks to Bill and sorry for the delay).

While resolving them, Bill found some simple ticket-0 changes that I 
applied based on his comments:

- removed the 20-year old outdated reference to EXPRESS
- slight wording change in advice to users on MPI_WEIGHTS_EMPTY
- slight wording change in example of graph_neighbors
- slight wording change in first sentence of 7.5.6

We can easily back them out (r2007) if we decide that we don't want 
them. They are rather uncontroversial though. The separate diff of these 
is at [1].

The complete diff (including all changes) is at [2] and the latest 
version of the chapter is at [3]. Please review this version again, 
hopefully a last time.

Thanks & All the Best,
	Torsten

[1]: http://htor.inf.ethz.ch/sec/chap-topol-diff-r2003.diff
[2]: http://htor.inf.ethz.ch/sec/chap-topol-diff-r1806.diff
[3]: http://htor.inf.ethz.ch/sec/chap-topol-mpi-3_1.pdf

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Torsten Hoefler           | Assistant Professor
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