def __init__(self, inputfiles, grid):
        """
        :param inputfiles: list of pdb files of aligned models
        :param grid: 2d-array coordinates of each point of a grid, fourth column full of zeros
        """
        self.inputfiles = inputfiles
        self.models = []
        self.header = []
        self.radius = None
        self.atoms = []
        self.grid = grid