default constructor defines sphere with radius 0.5mm and center (0,0,1)

class specifies ellipsoid tissue region (x-xc)^2/a^2 + (y-yc)^2/b^2 + (z-zc)^2/c^2 = 1 where center is (xc,yc,zc) and semi-axis along x-, y-, z- axes are a, b, c, respectively.