[WIP] Replace template metaprogramming by constexpr in LHCbMath

This is a new version of !6 (closed) re-adapted to the current status of LHCb.

From !6 (closed): Many template metaprogramming constructs can be replaced by the usage of constexpr now.

I changed Gaudi::Math::Digit(s) and pow in a way that there is no distinction between runtime and compile-time usage any more. The compiler will (hopefully) select what is most appropriate. My change potentially requires changes downstream. However, the only one I could see was in Gauss, for which I have a patch in hand.