Similarity measures

[foster999]

Hey, I have both a general question and feature request. I'm looking into similarity measures for spectra for a couple of use cases:

1. Comparing variation within a group of repeated measures of the same sample (measurement error) to the difference between samples
2. Comparing single spectra with a reference spectra, to filter out spectra that are too dissimilar to use in modelling (e.g. contaminated or partially saturated spectra)

Would you have any suggestions on measures for these two cases? I've found that Pearson coefficients are likely good enough for 2, but because most of our samples are relatively very similar it may not reflect finer differences in 1.

Then, are similarity measures something that you'd consider including in the package?

As an aside, I was really excited to find the chemtools package - I'd already implemented many similar transformers, but this will give us more options for grid searching etc. There are a few bits under Issues that I've worked on, so will create or add to pull requests where relevant