fix flip boolean bug in pulse_processing

[sophiafarrell]

An address to this issue: #265

Replaced (-1*flip) with np.where(flip, -1, 1) to fix flip=False bug in records flipping.
using np.where with flip as boolean, will return -1 if flip=True, else returns 1.
Previously boolean meant if flip=False d['data'] will go to 0.

Also I believe that calculating

strax/strax/processing/pulse_processing.py

Line 40 in 8e94cd9

samples_per_record = len(records[0]['data'])

in baseline computation is unnecessary.

[JelleAalbers]

Thanks for spotting Sophia! Setting all data to zero is not so good. I guess (-1 if flip else 1) or even (1 - 2 * flip) would work too in place of np.where, but up to you.

[sophiafarrell]

Yes, I like (-1 if flip else 1) the most, much simpler for people. I will edit it for that and thanks for the more elegant suggestion.