Rich - Use nested namespaces and 'if constexpr'

Adopt a few c++17 features

• Nested namespaces. So for instance replace

namespace A
{
   namespace B
   { 
       namespace C
       {
       }
   }
}

with

namespace A::B::C
{
}

• Simplify a number of places using SFINAE to provide scalar and SIMD specific function implementations with a single implementation that internally uses if constexpr.

Note there is a lot of whitespace change in this MR, due to the widescale change in indentation from the namespace changes.