CollapseCategory does not update when input data is not data.table

[boxuancui]

To re-produce:

data <- data.frame("a" = as.factor(round(rnorm(500, 10, 5))), "b" = rexp(500, 1:500))
table(data$a)
CollapseCategory(data, "a", 0.2, update = TRUE) ## data is not updated
table(data$a)

[boxuancui]

It is very hard to modify the input data directly, if it is not data.table. Here are a few modifications of this function:

1. update = TRUE will only work when input data is data.table.
2. You may still view the frequency distributions with any data format, as long as update = FALSE.