Peak integration improvements

[zeehio]

• In the tutorial, repeat peak detection after alignment and normalization and before integration. It improves results
• In the tutorial and docs, explain different strategies for peak integration and consequences.
  • Using the peaks from just one sample is a bad idea, unless that sample contains all possible peaks (pool of samples). In the case is a pool this may still not be the best approach.
  • Integrate peaks from all samples and merge peaks. We have room for improvement here in the pkg
• Show a plot with peak integrations
• Improve peak width estimation. Currently we base the estimation on the alanine peak, but we use the FWHM. I would probably 3/2 the FWHM so we integrate 90% of the peak, not just the top half (re-check the 3/2 based on a lorentzian distribution)
• Use wavelets for peak width estimation?