v2.1.5 CRAN release

RfEmpImp 2.1.5

• Add new wrapper function to recover dependency on ranger (>= 0.12.1) and
  to fix parameter passing
• New independent mice.impute.rfpred.norm function, and thus simplified
  mice.impute.rfpred.emp function
• Bypass collinearity and constant checks in mice for imp.* functions
• Add support for imputation using samples of classification trees predictions
  for prediction-based imputation (RfPred-Cate)
• Add new function query.rf.pred.idx for querying observation IDs associated
  with the terminal nodes
• Add new function query.rf.pred.val for querying observed values associated
  with the terminal nodes
• Add new function conv.factor for converting variables
• Update dependency for mice (>= 3.9.0) to avoid collinearity checks during
  imputation
• Update documentations