min_n: Create a function that return a result if number of valid obs...
In SESman/rbl: Biologging tools for diving predators in R

Create a function that return a result if number of valid obs >= n

1	min_n(f, n)

`f`	a function having a vector of data as first argument
`n`	the minimum number of observation required to return a non-NA result. A number in `(0;1[` will be interpreted as a proportion.

mean5 <- min_n(mean, 5)
mean5(c(1:4, NA))
mean5(c(1:5, NA))

mean90percent <- min_n(mean, 0.90)
mean90percent(c(1:8, NA, NA))
mean90percent(c(1:9, NA))