Set up a grid for the estimation of the quantile functions and CDFs
getGrid(target, controls, G)
A list containing the following elements:
grid.min
The minimum value of the grid
grid.max
The maximum value of the grid
grid.rand
A vector containing the grid points
grid.ord
A vector containing the grid points, ordered
A vector containing the data for the target unit
A list containing the data for the control units
The number of grid points