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