Multiple kinds of missing values

[nalimilan]

I've noticed the manual mentions that different kinds of missing values are currently not supported. This is indeed hard to do in Julia currently. I just wanted to let you know I've just started experimenting with this in a new TypedMissings.jl package. If you want to give it a try, it would be interesting to see whether it can allow representing all kinds of missing values from formats ReadStatTables supports.