add a new aggregator selection algorithm based on the performance
model described in:
Shweta Jha, Edgar Gabriel,
'Performance Models for Communication in Collective I/O Operations'
Proceedings of the 17th IEEE/ACM Symposium
on Cluster, Cloud and Grid Computing, Workshop on Theoretical
Approaches to Performance Evaluation, Modeling and Simulation, 2017.
Signed-off-by: Edgar Gabriel <gabriel@cs.uh.edu>
b3f59c76e1