Add "convert only compute constructs" option (and the alternative)

[domcharrier]

• Allows to convert only compute constructs and manually tune them while keeping all the other
  directives as directives in the code.
• The other directives can then be converted automatically by a preprocessor when
  compiling (add a separate option that excludes compute constructs).
• This would increase the maintainability of GPUFORT-translated applications.