Fix build error due to vtkStdString implicit conversion.
Fix build error due to vtkStdString implicit conversion.

"&myString[0]" is ambiguous for vtkStdString instances due to the
implicit const char* conversion method. Replacing with std::string.

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