adds flag for importance check
efficiency and consistency improvements

 - use dplyr
 - check obs. consistency using gw data
 - remove random 'na.omit's
 - use 'duplicated' instead of 'aggregate'
adds dplyr dependency
pipes!
bugfixes and improvements

 - use library so loading doesn't silently fail
 - no more auto-installation of packages using rstudio mirror
 - automatic coding of mi methods via rules
 - parallel mi via foreach
 - bugfix in lagging operation in data.R
 - include latent_sd
revisions to readme

in light of new changes