def createCells(self):
        '''Function to instantiate Cell objects based on the characteristics of this population'''
        # add individual cells
        if 'cellsList' in self.tags:
            cells = self.createCellsList()

        # create cells based on fixed number of cells
        elif 'numCells' in self.tags:
            cells = self.createCellsFixedNum()

        # create cells based on density (optional ynorm-dep)
        elif 'density' in self.tags:
            cells = self.createCellsDensity()

        # create cells based on density (optional ynorm-dep)
        elif 'gridSpacing' in self.tags:
            cells = self.createCellsGrid()

        # not enough tags to create cells
        else:
            self.tags['numCells'] = 1
            print('Warninig: number or density of cells not specified for population %s; defaulting to numCells = 1' % (self.tags['pop']))
            cells = self.createCellsFixedNum()

        return cells
Function to instantiate Cell objects based on the characteristics of this population