Revert "coll/basic: fix segmentation fault in neighborhood collectives if the degree"

This reverts commit 9c788ff9400960823637dc0eebb3a8640414fa05.
coll/basic: fix segmentation fault in neighborhood collectives if the degree
of the topology is higher than the communicator size

It is possible to have a topology degree higher than the size of the communicator.
For example, a periodic cartesian communicator on MPI_COMM_SELF. This will leave
the neighborhood collectives with a request buffer that is too small.

This commits introduces a semantic change :
from now, c_topo must be set before invoking coll_select
* comment on communicator creation in mca_topo_base_dist_graph_create(...)
* use accesors to retrieve topo info
Merge pull request #227 from ggouaillardet/rfc/coll_basic_neighbor

RFC/coll basic neighbor
Make sure initialization happens
Missed a removal from previous commit
Doh - if we can't output an entire block, then we need to adjust the number of bytes remaining to be output or else we will output duplicate bytes when next we are able to write.