returns convolution of vector a with vector b, a vector
of length na+nb-1 where na=numberof(a), nb=numberof(b).
In detail, for i=[1 to na+nb-1]
result(i) = sum j=[max(1,1+i-nb) to min(na,i)] (a(j)*b(1+i-j))
The n0= and n1= keywords can be used to control the section of
the full array that is actually returned, 1<=n0 line 6