einsum_py.py - prepare_op_axes(), change to original/left/right so I can better explore
alternatives. Add test case where 'left' fails

sop.pyx - cython code for sum_of_prod(); nops=2 case is modeled on the
ndinter sum_of_squares example.  It works, but does not have a speed advantage

compile cython with
# cython sop.pyx
# gcc -shared -pthread -fPIC -fwrapv -O2 -Wall -fno-strict-aliasing -I/usr/local/include/python2.7 -o sop.so sop.c