sd() on greta_array does not return a greta_array

Hi there, with the code below, applying `sd()` on a `greta_array` does not return the same class. Just wondering if it's intentional?

```
library(greta)
x <- normal(0, 1, dim = 10)
sd(x) # class numeric
mean(x) # class greta_array
```

I'm want to generate some calculated quantities by scaling using `sd(x)` so it'd be nice if it returns a `greta_array`. I could use `var(x)` or `scale(x)` (both return `greta_array`) but they are clunky in their own ways...

This was using `greta_0.4.1.9000` on `R version 4.1.3 (2022-03-10)`