ENH: 1st-order sections as 1st-order coefficients

Save a little bit of processing time by throwing away trailing zeros so the 1st-order sections are implemented as actual first-order filters, rather than wasting time multiplying by zeros:

```
x = randn(100000)
sos = array([[ 0.1667,  0.3333,  0.1667,  1.    ,  0.    ,  0.3333],
             [ 1.    ,  1.    ,  0.    ,  1.    ,  0.    ,  0.    ]])

timeit sosfilt(sos, x) # original
100 loops, best of 3: 3.12 ms per loop

timeit sosfilt(sos, x) # modified
100 loops, best of 3: 2.6 ms per loop
```

I think defining b and a makes the filtering lines more clear anyway