New collective: allreduce, fairly minimal communication.
New collective: allreduce, fairly minimal communication.

Note: this collective currently only supports 8-byte-sized types
to keep it simple.

uwsampa/grappa,uwsampa/grappa,buaasun/grappa,uwsampa/grappa,alexfrolov/grappa,alexfrolov/grappa,uwsampa/grappa,buaasun/grappa,uwsampa/grappa,alexfrolov/grappa,buaasun/grappa,alexfrolov/grappa,uwsampa/grappa,alexfrolov/grappa,buaasun/grappa,alexfrolov/grappa,buaasun/grappa,buaasun/grappa,alexfrolov/grappa,uwsampa/grappa,buaasun/grappa