Add impute() functions for imputation

[mnwright]

Example:

# Generate some missings
iris_na <- iris
for (j in 1:ncol(iris)) {
  iris_na[sample(1:nrow(iris), 5), j] <- NA
}

# Parallelization with doParallel
doParallel::registerDoParallel(cores = 2)

# Single imputation
iris_imputed <- arf::impute(iris_na, m = 1)

# Multiple imputation
iris_imputed <- arf::impute(iris_na, m = 20)