modified field classes

Hello,
There are changes related to mag. field:
1) separated chebyshev classes proper to MathUtil module (we are using them also for other applications)
2) the mfchebKGI_meas.root file is never used as a source of the field, we don't need it -> removed
3) the mfchebKGI_sym.root is rewritten with parameterization stored in the new AliceO2::Field::MagneticWrapperChebyshev objects - no need to use #pragma rules
Cheers,
 Ruben