* Bug 15393 fixed: nicholschart() fixed & improved

* Bug 15393 fixed: nicholschart() fixed & improved

  http://bugzilla.scilab.org/15393

  Updated page (PDF) http://bugzilla.scilab.org/attachment.cgi?id=5049

  * in a new figure, nothing was plotted
  * default cyan color was too flashy => gray85
  * colors could not be specified by their names or #RRGGBB or [r g b]
  * Positions of Gain labels were somehow puzzled
  * It was not easily to access to the vectors of handles for
     both subframes and for the set of labels.

  This commit complies with iscolor() extension @ https://codereview.scilab.org/21197

Change-Id: If2631b7fd2972dade04301ddbcc9e221ca34694f