disable MPI I/O by default (MPI_IO=n)

This is necessary because some resources -- such as
corbeil-mp2.rqchp.ca -- has problems with MPI I/O (or with the code
in Ray that uses MPI I/O).

Link: https://github.com/sebhtml/ray/issues/204
Link: https://github.com/sebhtml/ray/issues/206
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>