New Object: PixelPulse

Post-amplification pulse from the front-end, now dispatched as object. PixelHit carries reference to possibly existing PixelPulse. Can be used for waveform analysis and is a proposed solution to https://allpix-squared-forum.web.cern.ch/t/obtaining-time-dependent-signals-from-csadigitizer/290

Since it inherits from Pulse which is a descendant of std::vector<double>, the object can be directly looped over etc:

PixelPulse mypulse;
for(const auto& bin : mypulse) {
   // do something
}

The width of a time bin can be retrieved with getBinning(), units is framework units, i.e. ns.