title: When to use '.flat', '.flatiter' or '.flatten()'
body: <p>When is it appropriate to use which of the three flattening arguments '.flat'/'.flatiter'/'.flatten'? I know that '.flat' returns a 1D iterator over an array, does this mean that the array is left in the original shape and each element in the array can be accessed with a single index (for example using a single for loop even though the array may be highly dimensional). And '.flatten' returns a complete copy of the original array flattened out into a 1D array.</p>
  
  <p>Which is less resource intensive?</p>