Save first samples of peak(lets) waveform

[HenningSE]

What does the code in this PR do / what does it improve?

This PR uses the code changes in AxFoundation/strax#867 to save the first n_sum_wv_samples of not-downsampled waveforms.

[coveralls]

coverage: 90.168% (-0.08%) from 90.252%
when pulling 2f1a6af on add_waveform_start
into c2c0400 on master.

[dachengx]

Do we also need to add lone_hits to the start of waveform like

straxen/straxen/plugins/merged_s2s/merged_s2s.py

Line 140 in f514d5e

strax.add_lone_hits(merged_s2s, lh, self.to_pe, n_top_channels=n_top_pmts_if_digitize_top)

?

[HenningSE]

Do we also need to add lone_hits to the start of waveform like

straxen/straxen/plugins/merged_s2s/merged_s2s.py

Line 140 in f514d5e

strax.add_lone_hits(merged_s2s, lh, self.to_pe, n_top_channels=n_top_pmts_if_digitize_top)

?

Thanks @dachengx, yes we should add it! When checking the corresponding code I discovered a bug in strax that we should also fix: AxFoundation/strax#906

[HenningSE]

Needs AxFoundation/strax#908 to merge lone hits into the start of the waveforms

[yuema137]

Thanks @HenningSE ! On top of @dachengx's suggestions, I have a few comments

• The autotest would only work with Add option to merge lone_hits into data_start AxFoundation/strax#908 (that one looks good to me, and I already approved.), so please rerun the GitHub action after that PR is merged.
• Sorry that I may miss the context of this PR - is it widely needed by general analysis? I'm wondering if it's necessary to set the default value of save_waveform_start to True As @dachengx mentioned, we always want to compute the low-level plugins at once.

[dachengx]

Thanks @HenningSE ! On top of @dachengx's suggestions, I have a few comments

• The autotest would only work with Add option to merge LHs into data_start AxFoundation/strax#908 (that one looks good to me, and I already approved.), so please rerun the GitHub action after that PR is merged.
• Sorry that I may miss the context of this PR - is it widely needed by general analysis? I'm wondering if it's necessary to set the default value of save_waveform_start to be True

We can not process peaklets twice. So we'd better same everything during that only round of processing.