Urgent:
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1. finish new S4 classes/methods
2. update all documentation with new classes/methods
3. fix all related examples
4. update vignette

Interesting:
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1. PCA by depth slice
2. user-defined slices for profile_compare()
3. sensitivity analysis on profile_compare()
4. equal-area splines

Website:
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1. more examples
2. more figures
