Long live clang for pointing this out!

Standard CMAKE_CXX_FLAGS are a space-delimited list, but our
CXX_TESTS_FLAGS are manually passed to CUDA_ADD_EXECUTABLE, which
expects a semicolon-delimited list. Blame cmake for such a crazy
language...

- Use math function from TNL/Math.h instead of the standard library
- Many cosmetic changes for readability

- The inherited operator= is weird, but this seems to work.
- Use TNL::pow instead of std::pow etc.

The splitting with if( std::is_same<...> ) is not perfect, the code for
all devices was always visible to the compiler, and irrelevant parts
were optimized out at a much later stage. To fix compilation for MIC and
custom data type, we need proper template specializations.

- Cleaned up unnecessary specializations of TNL::max and TNL::min
- Implemented result type promotion to fix compilation with nvcc
- Added TNL::sqrt, which will be necessary for MIC

It used to work everywhere, but the compilers on CentOS (on penny)
are weird...