topoplot()   - plot a topographic map of an EEG field as a 2-D
                circular view (looking down at the top of the head) 
                 using cointerpolation on a fine cartesian grid.
 Usage:
        >>  topoplot(datavector,'eloc_file');
        >>  topoplot(datavector,'eloc_file', 'Param1','Value1', ...)

 Inputs:

    datavector = vector of values at the corresponding locations.
   'eloc_file' = name of an EEG electrode position file {0 -> 'chan_file'}

 Optional Parameters & Values (in any order):

                  Param                         Value

                'colormap'         -  any sized colormap
                'interplimits'     - 'electrodes' to furthest electrode
                                     'head' to edge of head
                                        {default 'head'}
                'gridscale'        -  scaling grid size {default 67}
                'maplimits'        - 'absmax' +/- the absolute-max 
                                     'maxmin' scale to data range
                                     [clim1,clim2] user-definined lo/hi
                                        {default = 'absmax'}
                'style'            - 'straight' colormap only
                                     'contour' contour lines only
                                     'both' both colormap and contour lines
                                     'fill' constant color between lines
                                     'blank' just head and electrodes
                                        {default = 'both'}
                'numcontour'       - number of contour lines
                                        {default = 6}
                'shading'          - 'flat','interp'  {default = 'flat'}
                'headcolor'        - Color of head cartoon {default black}
                'electrodes'       - 'on','off','labels','numbers'
                'efontsize','electcolor','emarker','emarkersize' - details

 Note: topoplot() only works when map limits are >= the max and min 
                                     interpolated data values.
 Eloc_file format: 
         chan_number degrees radius reject_level amp_gain channel_name
        (Angle-0 = Cz-to-Fz; C3-angle =-90; Radius at edge of image = 0.5)
        For a sample eloc file:    >>  topoplot('example')

 Note: topoplot() will ignore any electrode with a position outside 
       the head (radius > 0.5)

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