Allow the vertical dimension to not be specified for MPAS.
Allow the vertical dimension to not be specified for MPAS.

A missing vertical dimension will no longer cause a file load to fail.

keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,sumedhasingla/VTK,keithroe/vtkoptix