Relax team::split() uniqueness precondition

[Dan Bonachea]

Current preconditions for team::split ( color , key ) include:

No two ranks in the collective call may specify the same combination of color and key.

This precondition is stronger than both MPI_Comm_Split and gex_TM_Split, both of which explicitly allow such inputs and specify that "ties" are broken according to rank in the parent team.

I think this precondition will be surprising to MPI programmers, and in some cases it may require the program to needlessly execute additional collective operations before the split to establish the uniqueness needed to honor this pointless precondition.

Proposal:

1. Remove the precondition.
2. Add a sentence stating that "ties" are broken according to rank in the parent team.

[Dan Bonachea]

Resolved in 2d9680f (pull request #16)