[ferret_users] contour/levels extension?

[Ryo Furue <furue@xxxxxxxxxx>]

Ferret users and developers,

I find the "LEVELS=x.xD" qualifier handy, but I thought
it would be even nicer if we had some of the additional
capabilities of the "LEVELS=(lo,hi,delta,ndigits)"
qualifier.  For example, I sometimes want to use "LEVELS=x.xD"
but don't want contour labels.

So, I thought it might be useful to extend
the "LEVELS=(lo,hi,delta,ndigits)" qualifier to allow
things like

   LEVELS=( , , 0.1, -3)

which is supposed to be equivalent to "LEVELS=0.1D" but
with contour labels omitted.  Would this be feasible as
an extension?

(As an aside, it doesn't trigger error in the current Ferret
 implementation, but it doesn't have an anticipated result.)

I also think something like

   LEVELS=( , , 0.1C)
   LEVELS=(-3.3, 4, 0.5C)

would be handy to ensure zero contours are included.
(You might want to use the latter in a script where
 the lo and hi of the contour levels are supplied
 as arguments.)

Cheers,
Ryo