consolidate filter methods

[atspaeth]

Relating to the discussion with @surygeng in #41, also attn: @ashsrobbins.

• Deprecate filter() for butter_filter()
• Simplify filtering SpikeData.from_thresholding()

The first commit here is just deprecating the more complicated filter method whose fancy features it doesn't look like are being used.

The second is breaking, so let's reject this if people are actually using this feature: the interface of the SpikeData.from_thresholding() constructor was overly complicated, with 4 different kwargs being used for the optional pre-filtering. This is now simplified by just having one parameter filter that gives the parameters for butter_filter(), and can be None to emulate the previous do_filter=False behavior.

[DailyDreaming]

This looks good to me. If inclined, I'd add a test, but otherwise I think this is good to merge.