Jonswap spectrum not normalized with cutt off frequency

[yoeri van cauteren]

I was looking at the matlab files of jonswap.m and and mkjonswap.m and
noticed that the options.wnc variable is never defined past the
default initialization of 6. Also this default is not entirely the
same as the default value of the 33/Tp set for wc. The more correct
value is 2*Pi*6 being 37.7/Tp.

As it is written now: the cut off frequency defines the range of the
spectrum but not the interval of its normalization. Now normally this
isn't much of a problem when the cut of limit is big enough but I
needed it to be smaller and then the difference in the interval of
normalization becomes important. So I simply added the line:
"options.wnc = wc/2/pi*Tp;"
to the jonswap.m file as one point in time this was apparently already
foreseen but never implemented. Some may like the way it functions now
but I think an option of including this would be nice.

I don't know if its worth mentioning but I thought I'd let you guys
know.

Kind regards