finished first pass on Python tutorials (cf. #795)

demos/api/explore.cpp

tutorial/c++/t1.cpp

tutorial/c++/t10.cpp

tutorial/c++/t11.cpp

tutorial/c++/t12.cpp

tutorial/c++/t13.cpp

tutorial/c++/t14.cpp

tutorial/c++/t15.cpp

tutorial/c++/t16.cpp

tutorial/c++/t18.cpp

tutorial/c++/t2.cpp

tutorial/c++/t3.cpp

tutorial/c++/t6.cpp

tutorial/c++/t7.cpp

tutorial/c++/t8.cpp

tutorial/c++/t9.cpp

tutorial/c++/x1.cpp

tutorial/c/t1.c

tutorial/python/README.txt

tutorial/python/t1.py