iFit/sw_quadell

 calculates and plots the parameters of an ellipsoid from a quadratic form

 ellM = SW_QUADELL(M, plot)

 It calculates the parameters of an ellipsoid belonging to the input
 quadratic form and plots it. Can be used as a tool to visualise
 anisotropy matrices, g-tensors, etc.

 Input:

 M         Quadratic form coefficients in a 3x3 matrix.
 toplot    If true, the ellipsoid is plotted. Default is true.

 Output:

 ellM      Contais the principal axis of the ellipsoid in the columns, 3x3
           matrix.

 On the plot the principal axes are shown by a red line, the x,y and z
 axes are shown by black line.