When using Cantera from a GUI app, the python process was inheriting
window settings from the parent, and launching in a window that didn't
accept input from the calling process, causing the conversion to fail.
Added libstringstream, which maps stdin and stdout to std::iostream.
Using this library means we no longer create either the temporary .py
file or the ct2ctml.log file.
This also eliminates the synchronization problems that motivated the
sleep commands around the system call, so cti to ctml conversions are
significantly faster now.