File iterator increment operators should be consistent with STL

[jmthibault79]

Specifically, preincrement operators should return a reference to the iterator, not the value. Where applicable (copyable), postincrement operators should return a copy of the iterator before the increment operation.

Inspired by #375

[kgururaj]

Same for decrement operator also

[jmthibault79]

Yes, if we have any decrements. But we don't in the file iterators. I'll update the ticket name to clarify that I only mean those iterators.