Guard against swapped indices

fe_18.wgfa contains transitions with swapped lower and upper indices.

The previous implementation assumed the order was correct, and checked
only that the upper level index was below any applied level cut.  The
Chianti bug was thus preserved in the script's output.

Cloudy crashed on fe_18, when ran with bounds-checking on.

Here, a guard is added against swapped indices in .wgfa files, and
preemptively in .scups files.