Add a filter function for raw data

[surygeng]

Add a butterworth filter function for getting LFP from raw data.

[pupster90]

Looks good, this has the filter function in it

[pupster90]

does this merge?

[atspaeth]

What's the difference between this and the one that's just called filter()? (Added by @ashsrobbins a while back.) Can we consolidate the two?

[surygeng]

This new function is simpler. It can be lowpass, highpass, or bandpass, depending on the cutoff value. It returns the same shape of data as the input. Also, the scipy sosfiltfilt function is used since it performs well over other filters. Frequency response was checked.

[atspaeth]

Great, so the only pro of the old one is that it's chunked/incremental. @ashsrobbins do we actually use that? Is it faster in practice on sufficiently large data or something?

Otherwise I can put together a PR that deprecates the old one and uses the new one in SpikeData.from_thresholded()