Remove use of include <vtksys/ios/sstream> and vtksys_ios::*stringstream
Use the following commands to switch to standard header and namespace:
git grep -l vtksys/ios | xargs sed -i 's|vtksys/ios/sstream|sstream|' git grep -l vtksys_ios:: | xargs sed -i 's|vtksys_ios::|std::|g'
