Calculate the moving values

Usage

mov(x, k = 10, shift = "centre", fun = "mean")

x: A vector to calculate moving values
k: The moving windows
shift: if shift = "centre", then values are shifted to centre. if shift = "begin", then values are at begin of period. if shift = "end", then values are at end of period. The default value (centre) will be used if shift is other value.
fun: The method to calculate moving values. Curruntly, only "mean", "max", "min", and "sum" are supported. A NULL will be returned for any other values

The moving value of vector x at moving windows k. A NULL will be returned for any unsupported fun