Use constexpr for physical constants and move them into gmx namespace

Use constexpr for physical constants and move them into gmx namespace

The physical constants that are part of the public API are converted from
defines to constexprs and are moved into the gmx namespace. This means
that many files are touched with tirvial changes. The benefit is that
now the risk of symbol collision is reduced in cases where a library
might depend on these constants.

Additionally, in the same vein of library cleanup, the math/utility
header is removed from units header and included only in the sources
that actually need it.