How to sort rows in a `D2Array` by value in the last (or nth) column

I noticed a `D2Array` is just a single array containing all elements, instead of a nested array of arrays.

So how can the "rows" be sorted by the values of the last (or nth) column.

The equivalent in plain Kotlin would be:

```kotlin
val foo = Array(10) { DoubleArray(10) { nextDouble() } }

foo.sortBy { it.last() }
```

The issue names no source file or test. Start by locating D2Array's flat storage and any existing row-access or sorting APIs, then determine how sorting by the last or nth column should be exposed. Done means the requested row-ordering behavior is implemented or documented with coverage for the relevant cases.

Written by the indexing model from the issue text.