MPI_File_preallocate is slow

[tkphd]

MMSP.grid.hpp calls MPI_File_preallocate before writing data at specified offsets, to make sure the filesystem has allocated sufficient space for the incoming data. However, profiling with mpiP reveals that this preallocation is a bottleneck. This is explained in "On Implementing MPI-IO Portably and with High Performance": most filesystems do not implement a preallocation routine, so MPI-IO must write data to disk in order to force allocation.

This reflects a minor design flaw, in that MMSP forces something the filesystem clearly does not require. Therefore, it should be safe to drop MPI_File_preallocate from the grid class. Further, it may be desirable to use the collective MPI_File_iwrite_at_all instead of non-collective MPI_File_iwrite_at to perform the file output.