Variable-cell simplification

In variable-cell calculations, G-vectors and related quantities must be computed with respect to the initial cell. When a calculation is restarted, the cell is read from input, read from data, swapped and swapped again, and one is never sure what is "at" and what is "at_old". After this commit the initialization of the G-vectors depends upon a single cell variable, "ref_at", already existing because used in CP. BEWARE: may have unexpected consequences. TODO: remove unnecessary swapping between "at" and "at_old"