Modernize LHCbMath

• fix return type of 'abs_less' and 'abs_greater' (matching std::less and std::greater)
• use perfect forwarding in 'abs_less' and 'abs_greater'
• provide default template argument for 'abs_less' and 'abs_greater' (matching std::less and std::greater)
• prefer 'begin' and 'end' over & of first and & of last-1
• remove obsolete SVN/CVS identifiers