add pure, impure, elemental instructions
#### Context
Introduce the use of pure, impure, elemental instructions for performance issues
- [x] All configurations
- [x] Modifications of versioned files: AGRIF and Fortran routines
#### Proposal
- [x] We must first add pure, impure and elemental in AGRIF grammar
- [x] Next introduce them in NEMO routines and test the new perfs...
issue