Modify ParaViewDataCollection restart mode
Modify ParaViewDataCollection restart mode

* Make more portable: don't depend on truncate from unistd
* Verify that the cycle we're trying to write won't overwrite any previous cycle
we are retaining

mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem