magnetometers calibration

MgnCalibration performs magnetometer calibration from a set of data using Merayo technique with a non iterative algoritm J.Merayo et al. "Scalar calibration of vector magnemoters" Meas. Sci. Technol. 11 (2000) 120-132.
This implementation use svq, qr transformation and cholesky factorization.
The method tries to find the best 3D ellipsoid that fits the data set and returns the parameters of this ellipsoid (shape U and center c). The Ellipsoid equation is : (v-c)'*(U'*U)(v-c) = 1
with v a rough triaxes magnetometer measurement.
The calibrated measurement is given by w = U*(v-c)
The algorith must operate upon a set of data which describes
as completely as possible the 3D space.
Because the svd performs an optimzation fit without constraints, (as does the original iterative Merayo method's), the algorithm may fail with too poor data.