label.factor: Transform a variable into factor
In siyuany/autobin: Variables Binning for Scorecards Development

These functions turn a variable into a factor, with NA as an independent level called 'Missing'.

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label.factor(x, levels = NULL)

label.numeric(x, cuts, single.values = NULL, na.level = NULL)

`x`	a numeric or a vector which can be coerced to be a factor
`levels`	designated levels in the result factor
`cuts`	cut points to segment a numeric, using a right-close interval
`single.values`	For numeric vectors, some of the values should be as separate levels.
`na.level`	should NA be an independent level? NULL for default behavior which inducts