Change VTK file extension when running in parallel.
Change VTK file extension when running in parallel.

Paraview expects a .pvtu extension in this case instead of .vtk.

Refs libMesh/libmesh#882.

SudiptaBiswas/moose,harterj/moose,bwspenc/moose,yipenggao/moose,dschwen/moose,laagesen/moose,yipenggao/moose,jasondhales/moose,jasondhales/moose,dschwen/moose,jessecarterMOOSE/moose,stimpsonsg/moose,bwspenc/moose,backmari/moose,yipenggao/moose,milljm/moose,friedmud/moose,liuwenf/moose,andrsd/moose,nuclear-wizard/moose,jessecarterMOOSE/moose,milljm/moose,lindsayad/moose,lindsayad/moose,sapitts/moose,andrsd/moose,SudiptaBiswas/moose,Chuban/moose,katyhuff/moose,bwspenc/moose,permcody/moose,nuclear-wizard/moose,joshua-cogliati-inl/moose,friedmud/moose,katyhuff/moose,liuwenf/moose,idaholab/moose,stimpsonsg/moose,sapitts/moose,idaholab/moose,yipenggao/moose,backmari/moose,joshua-cogliati-inl/moose,milljm/moose,lindsayad/moose,sapitts/moose,Chuban/moose,katyhuff/moose,backmari/moose,laagesen/moose,permcody/moose,jasondhales/moose,nuclear-wizard/moose,idaholab/moose,friedmud/moose,SudiptaBiswas/moose,harterj/moose,jhbradley/moose,stimpsonsg/moose,harterj/moose,katyhuff/moose,liuwenf/moose,friedmud/moose,permcody/moose,jhbradley/moose,bwspenc/moose,lindsayad/moose,permcody/moose,joshua-cogliati-inl/moose,jasondhales/moose,jessecarterMOOSE/moose,backmari/moose,andrsd/moose,liuwenf/moose,harterj/moose,Chuban/moose,idaholab/moose,bwspenc/moose,nuclear-wizard/moose,SudiptaBiswas/moose,YaqiWang/moose,YaqiWang/moose,sapitts/moose,joshua-cogliati-inl/moose,dschwen/moose,laagesen/moose,jhbradley/moose,dschwen/moose,YaqiWang/moose,jessecarterMOOSE/moose,stimpsonsg/moose,liuwenf/moose,harterj/moose,lindsayad/moose,jhbradley/moose,idaholab/moose,laagesen/moose,jessecarterMOOSE/moose,YaqiWang/moose,sapitts/moose,laagesen/moose,Chuban/moose,liuwenf/moose,dschwen/moose,milljm/moose,SudiptaBiswas/moose,milljm/moose,andrsd/moose,andrsd/moose