Set the elements of the matrix im which are in the interior of the
ellipse E to the value 'c1'. The ellipse E has center (y0, x0), the
major axis = a, the minor axis = b, and theta is the angle made by the
major axis with the orizontal axis.
ret = ellipseMatrix(y0, x0, a, b, theta, im, color)
ret is a matrix of the same size as the matrix im.
The input parameters c2 and pInterp are optional. If they are present
the ellipse will have a smoother border (of size pInterp pixels) with
variations of color between c1 and c2.
% October 14, 2014
% Author: Nicolae Cindea