[daip] IMAGR. UVWTFN, ROBUST

[Eric Greisen]

The wording of the help file was confusing.

If N is the first character then natural weighting is done, otherwise
uniform is done.  Uniform uses ROBUST and the other adverbs UVSIZE,
UVBOX, UVBXFN.

The 2nd character modifies the weights before they are used in
natural or uniform.  Additionally, a first character of C modifies the
weights by taking 1 over the weight before it is used.  Why that is
there is anyone's guess.

Eric Greisen