Added new array templates functions to know if first or last array item is displayed

Sometimes when I customize a afArrayField, I would like to know if I display the first or the last afEachArrayItem.

What do you think about adding these two new templates in autoform-helpers:
- afArrayFieldIsFirst
- afArrayFieldIsLast

I am not quite sure about the currentField parameter. May be it is better to pass the field name to retrieve later the field object?
I hope it can help