DEFAULT_MARKERS Return the list of default markers

 Return N markers based on pre-defined categories
     [ML,CL]=DEFAULT_MARKERS(N)

 Generate random markers
     [ML,CL]=DEFAULT_MARKERS(N,'random')

 INPUT
    N    number of classes

 OUTPUT
    ML   marker list (cell array with marker strings)
    CL   color list (cell array with color vectors)

 DECRIPTION
 DEFAULT_MARKERS is used to populate marker list property of newly
 created data sets.
 You may change this function to provide custom markers and colors.