Maintain API compatibility by renaming a few modules

We can maintain compatibility in the API (e.g. keep 'gridxc_cellxc' instead of 'cellxc_gridxc') by renaming a few modules:

gridxc_atomxc --> gridxc_atom gridxc_cellxc --> gridxc_cell gridxc_ldaxc --> gridxc_lda gridxc_ggaxc --> gridxc_gga