fast pmm and an auto function

• fill_NA_N has a new model which is pmm - predictive mean matching
• fast PMM - presorting and binary search
• naive_fill_NA - auto function for data.frames - bayes mean and lda
• ridge argument for lm models - adding small disturbance to diag of X'X
• lm_bayes provide more disturbance
• new tests
• codecov

comfortable environment for data.table/dplyr users

• providing a more comfortable environment for data.table/dplyr users
• expand vignette and documentation
• updated performance benchmarks
• fix a glitch - e.g. lack of correct warning for a lda model with zero variance variables