STK_LEGEND create a legend automatically

 CALL: stk_legend ()

   creates a legend automatically, using only the graphical objcts for which a
   non-empty DisplayName has been provided.

 CALL: h = stk_legend ()

   also return a handle to the legend (which is either a numerical handle or a
   Legend object, depending on the version of Matlab/Octave that you are
   using).

 NOTE:

   Creating and displaying a legend is painfully slow in some versions of M/O.
   You should *really* avoid doing that inside a loop if you want a reasonably
   fast update of the display.

 See also: stk_plot1d