nObs needs to be at start of data block

[aylapear]

The output of data block needs to have nObs at the start.

Current output:

> mod_data <- nlist::as_nlist(list(
+   X = c(1L, 2L, 3L, 4L),
+   nObs = 4L
+ ))
> x <- data_block(mod_data)
> cat(x)
data {
  int X[nObs];
  int nObs;
}

Needs to be

> mod_data <- nlist::as_nlist(list(
+   X = c(1L, 2L, 3L, 4L),
+   nObs = 4L
+ ))
> x <- data_block(mod_data)
> cat(x)
data {
  int nObs;
  int X[nObs];
}