ue] determines output type (faster if useSingle)

 OUTPUTS
  J          - [hxwx3] single or double output image (normalized to [0,1])

 EXAMPLE - luv
  I = imread('peppers.png');
  tic, J = rgbConvert( I, 'luv' ); toc
  figure(1); montage2( J );

 EXAMPLE - hsv
  I=imread('peppers.png');
  tic, J1=rgb2hsv( I ); toc
  tic, J2=rgbConvert( I, 'hsv' ); toc
  mean2(abs(J1-J2))

 EXAMPLE - gray
  I=imread('peppers.png');
  tic, J1=rgb2gray( I ); toc
  tic, J2=rgbConvert( I, 'gray' ); toc
  J1=