Add option to save first samples of peak(lets) waveform

[HenningSE]

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

This PR adds the option to save the first n (default to 200) samples of the merged peak(lets) waveform without downsampling. In case of very wide S1s or S1s with a lot of afterpulses strax will downsample the peak waveforms to fit into the 200 sample summed waveform that we save with peaklets and peaks. This process removes important information of the S1 waveform. With this PR the first n samples of the waveforms are saved for peaklets that 1. get downsampled and 2. have a downsampling factor smaller 7. This should cover basically all S1s without introducing a large file size overhead.