Modernize MagneticField code

• remove unused MagMat,MagVec,AnalyticFieldSvc
• amalgamate component headers into their source files
• rename MagneticFieldGrid::fieldVector to MagneticFieldGrid::fieldVectorLinearInterpolation (to be able to add other interpolation schemes in the future)
• prefer Gaudi::Property
• prefer auto over ...::iterator
• prefer range-based for loops
• rename m_Q_V into more obvious m_B
• prefer warning(),info(),debug() over explicit MsgStream construction
• remove redundant explicit destructors

must be applied in conjunction with Rec!2042 (merged)