Issue 52

Creates a new method _get_cells_poly_simple to replace _get_cells_poly. This method determines the source grid cell geometry by using the mid-point between coordinate nodes as a faster alternative to the existing method which calculates the centroid of each of cells surrounding a grid node, which is more general, but since we are uniformly working with 1D projected coordinates, this more simple method will suffice and should be much faster, especially for larger source grids.