[OpenCL] Enhancements suggested

[kif]

Some suggestion from Loic, Wout, Olof, Alex, Armando ...

1. Summarize the many programming languages into a table, will be faster and easier for the comparison.
2. Performance portability of the code
3. Emphasize on (long term) maintainability and the relation with metaprogramming
4. Maturity
5. show the generated code to prove it is readable (?) remove the black-magic part.