Computes the average missing value gap of a vector.

## Arguments

- x
a numeric vector

- what
default is the arithmetic mean.
One can include an own function that returns a vector of lenght 1 (e.g. median)

## Details

The length of each sequence of missing values (gap) in a vector is calculated and the
mean gap is reported

## Author

Matthias Templ based on a suggestion and draft from Huang Tian Yuan.

## Examples

```
v <- rnorm(20)
v[3] <- NA
v[6:9] <- NA
v[13:17] <- NA
v
#> [1] -0.87231588 0.10668461 NA -0.32785359 -0.08536101 NA
#> [7] NA NA NA -0.55323819 1.40510888 -0.79546095
#> [13] NA NA NA NA NA -0.07681765
#> [19] 0.89292492 -0.77750309
gapMiss(v)
#> [1] 3.333333
gapMiss(v, what = median)
#> [1] 4
gapMiss(v, what = function(x) mean(x, trim = 0.1))
#> [1] 3.333333
gapMiss(v, what = var)
#> [1] 4.333333
```