Let `elems` argument of `rownth()` expand to corresponding length if length is 1

[psychelzh]

For example:

set.seed(123)
x <- matrix(rnorm(1000), nrow = 10)
Rfast::rownth(x, 1)
#> [1] -2.809775
# expect this instead:
Rfast::rownth(x, rep(1, nrow(x)))
#>  [1] -2.809775 -2.309169 -1.943651 -1.779977 -2.249051 -2.660923 -2.014210
#>  [8] -2.070751 -2.465898 -2.078489

^{Created on 2024-04-15 with reprex v2.1.0}

[ManosPapadakis95]

I will think about it. The reason i didn't do it is because i want be clear that each cell of that argument corresponds to each row.