remove most namespace aliases to prepare for #813

tutorial/c++/t10.cpp

tutorial/c++/t12.cpp

tutorial/c++/t13.cpp

tutorial/c++/t14.cpp

tutorial/c++/t15.cpp

tutorial/c++/t16.cpp

tutorial/c++/t17.cpp

tutorial/c++/t2.cpp

tutorial/c++/t3.cpp

tutorial/c++/t4.cpp

tutorial/c++/t5.cpp

tutorial/c++/t6.cpp

tutorial/c++/t8.cpp

tutorial/julia/t16.jl

tutorial/julia/t2.jl

tutorial/julia/t3.jl

tutorial/julia/t4.jl

tutorial/julia/t5.jl

tutorial/python/t10.py

tutorial/python/t11.py