Urgent:
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1. fix SD calculation in soil.slot() when using user-defined segment size
2. fix wt. mean and SD calculations in soil.slot()

Interesting:
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1. PCA by depth slice
2. user-defined slices for profile_compare()
3. objective selection of k
4. sensitivity analysis on profile_compare()
5. equal-area splines
6. check profile-level weighting scheme

Website:
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1. more examples
2. more figures