simplify MPI_grid class
simplify MPI_grid class

electronic-structure/sirius,electronic-structure/sirius,toxa81/sirius,electronic-structure/sirius,electronic-structure/sirius,toxa81/sirius,toxa81/sirius,toxa81/sirius,toxa81/sirius,electronic-structure/sirius,electronic-structure/sirius,electronic-structure/sirius,toxa81/sirius