Allow deactivating MPI I/O in checkpointing
As reported in #73 (closed) MPI I/O can lead to segfaults in some cases. Using openmpi-4.1.4 (version 3.1) I encountered incorrect values which have been stored using MPI I/O. This problem was hard to detect especially since MPI I/O is activated by default. With this MR the default is to deactivate MPI I/O and a user can activate it using a keyword argument.