Rename nnz() to nnz_sum() (in matlab and py wrappers).

misc/demo/Quick_start/quick_start.m

misc/test/src/Matlab/FaustTest.m

misc/test/src/Matlab/test_matlab_faust.m

misc/test/src/Python/test_FaustPy.py

wrapper/matlab/Faust.m

wrapper/python/FaustPy.py

wrapper/python/quickstart.py