FIX: Add initial conditions

Initial conditions now work. Note that the `zi` that `sosfilt` expects are *not* the same initial conditions that `lfilter` expects, because the filter has a different form. Eventually we might want to add an `sosic(sos, k, y, x=None)` function to provide equivalent functionality to `filtic`, but I couldn't figure out the math. (I think it will require a state-space analysis, which I'm too rusty on at the moment.) In any case, the basic tests I put in pass. Docs will need updating eventually, I left them blank in a couple of places.