Fix unused variables by adding a MATH_UNUSED macro in Math/Util.h
This fixes problem with documentation reported in https://savannah.cern.ch/bugs/?100622
Add also an error message for the not implemented methods


git-svn-id: https://root.cern.ch/svn/root/trunk@49063 27541ba8-7e3a-0410-8455-c3a389f83636