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