This Function give a way to use MPIIO transparently. It provide a way to
write a file from distributed string (or osstream). The strings are
written to file in ascending process id order.
The string provided to the function by proc 0 is written first in the
generated file. Then the one hold by proc 1 is written. And so on. All
process are not obliged to provide a non null string but they must all
participate to the call to this function even with empty string.

An atomic test illustrate this function usage and shows how multiple
files generation can be used to improve IO throughput. With large number
of process H. Digonnet (ICI) experienced that multiple file output with
adhoc communicator is better then one unique file with MPIIO (to be
checked in our context with this test case enlarged to more
data/process).

A priori there is no limitation to string size except memory
consumption. This function might be used in the future for xExport
library.

Note that initially this function was added to xExport library. But this
library is dependent of xFem so it have been  introduced here in xTool.