Use MPI_Waitall instead of MPI_Test polling.

Just checking id this is faster or slower than the async polling version. It is a little slower, but not a large amount.

README.md

 SWIFTmpistepsim
 ===============
 
+Variation: this branch uses an `MPI_Waitall` call to synchronously wait
+for all the available requests, rather than polling using MPI_Test.
+The result runs slightly slower, but takes roughly similar times as
+the MPI_Test version.
+
 This project is a standalone part of [SWIFT](http://www.swiftsim.com) that
 aims to roughly simulate the MPI interactions that taking a single step of a
 SWIFT simulation makes. Making it possible to more easily see the performance