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)

* determine optimal level of checking in validity method for SoilProfileCollection objects

* make slab() faster / more efficient

* slab() no longer supports weighted-aggregates

* integrate color styling with 'scales' package in plot.SoilProfileCollection

* more aggressive checking of ordering / IDs (especially rbind.SoilProfileCollection)

* Should we move to reshape2? It is faster. (PR)

* S4-cleanup of profile_compare

* raster::extract method

* sp::over method

* rgdal::spTransform method


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

