Urgent:
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** ADD to all modules using data.frame, melt, ... ?
  ## important: change the default behavior of data.frame and melt
  opt.original <- options(stringsAsFactors = FALSE)
1. slab() breaks when use.wts=TRUE --> generalize and re-factor related code
2. integrate color styling with 'scales' package in plot.SoilProfileCollection
3. more aggressive checking of ordering / IDs
4. Should we move to reshape2? It is faster. (PR)
5. S4-cleanup of slice / slab / profile_compare
6. concat methods for SPC objects

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