Define a contoured region-of-interest of an image using the image
display system and the cursor/mouse. This function returns a vector
containing the subscripts of pixels inside the contoured region. Note
that these are `linear' subscripts, not `X, Y' pairs.
IDL> result = conroi( data(*,*,n) )
IDL> result = conroi( data(*,*,n), zoom=4)