Add macros to make codelets just declare their access and let the macro definition do what should be

Here is written down the set of generic macro I'm thinking of to make codelet declare their memory access (see !16 (closed))

For now they are very trivial, and produce exactly the same effect as with the current master. But then one can easily add the morse_desc_iscached() test, add statistics, etc. without having to change the codelets again.

I've put it in control/descriptor.h for simplicity, but we can of course push to runtime-specific code (which file?)