Implement mapping functions

[kubakl]

I would like to have a clean way of sanitising my csvs before the unmarshaling happens. Take this csv as an example:

Name,Age,Height
John,10,1.80
Jack,20,N/A

In this case it will fail to decode due to N/A value. I would like to have a way to replace it with NaN or an empty string so it's omitted. The solution from csvutil package is a really easy and clean one.

	dec, err := csvutil.NewDecoder(r)
	if err != nil {
		log.Fatal(err)
	}

	dec.Map = func(field, column string, v any) string {
		if _, ok := v.(float64); ok && field == "n/a" {
			return "NaN"
		}
		return field
	}