simplifying filtertools

[nirum]

In filtertools, we have three functions (getsta, getstc, getste) that are 90% the exact same. I'm thinking that we can reduce these into a single function that selects out the spike-triggered stimuli and into an iterable sequence and then collapsing the sequence down as necessary for the STA and STC, or returning the whole sequence for the STE.

[nirum]

💥 got rid of 275 lines of code.

Now, filtertools.getste returns an iterator (via itertools.imap) that iterates over each spike-triggered stimulus slice.

Then, filtertools.getsta and filtertools.getstc simply use the built-in reduce function to collapse the iterator into either the STA or STC matrix.

I tested this with toy data in an ipython shell, and it seems to work well. This still needs to be tested with stimuli stored as hdf5 arrays, but I think this method should solve Issue #48 as well.

😄 👍 🎉