Very useful piece of code!
evecs is eigenvectors matrix. Following the principal axis theorem, eigenvectors represent principal axes of ellipsoid. Regarding use with magnetometer calibration, those are needed to perform so called "soft iron" calibration, in order to properly shrink the ellipsoid into sphere. I would recommend taking a look at freescale's AN4246 & AN4248.
11 Mar 2014
Fits an ellipsoid to data using linear least squares.