Reverted VTK change about std::string + vtkOStreamWrapper.

There is some confusion about string handling, this patch
works around the issue.

Change-Id: Ie2161488d9fcbab26f085048950f70562f67b5f0
Merge topic '14614-report-numpy' into next

71d58a4b Reverted VTK change about std::string + vtkOStreamWrapper.