smurf: Change soft-edged filter to use Butterworth filter

The flt.filt_edgewidth parameter has been replaced with flt.filt_order.
If flt.filt_order is zero (the default), a hard-edge filter will be used.
Otherwise, flt.filt_order gives the order of a Butterworth filter to be
used. Higher order filters produce sharper cut-offs, which give better
frequency response but worse ringing. A hard-edged filter is equivalent
to a very high order Butterworth filter.