Option to give range to sum_waveform

[zhut19]

What is the problem / what does the code in this PR do

The desaturation requires re-calculating the sum waveform of only a small fraction of the peaklets (that are saturated), once the (temporary copy of) records are corrected. It is best to be able to do partial re-calculation.

Can you briefly describe how it works?

func: sum_waveform now takes a keyword argument peak_list of the type numpy.array. When not specified, default value would be converted into np.arange(len(peaks))

[JoranAngevaare]

Hi @zhut19 , thanks for the PR, having the desaturation correction in strax will be most helpfull.

In this PR, the style seems very convoluted. Can you explain why you need -3 and -14? I think you can more easily and clearly do this with a simple None kwarg?

By the way, travis is failing untill we merge #321. Don't worry about that too much.

[zhut19]

Thanks, @jorana. I wasn't thinking. The keyword argument was one way to keep compatibility with the original code, a silly one tbh. With the switching to *arg in 6aeb711, is this good for you?

[JoranAngevaare]

Sure, would work! But if at all possible I'd be a little more explicit, would you like 6275049? Otherwise I can revert it?

[zhut19]

This is even better! I remove the raise error, as an empty list is okay and does exist.

[JoranAngevaare]

Sure thanks, I was about to remove that raise statement for exactly the reason you mention. Just for completeness I've added this to the tests in f92086f