code: def positionedcrop(self,x,y,sheet_coord_system):
          
          cf_row,cf_col = sheet_coord_system.sheet2matrixidx(x,y)
          bounds_x,bounds_y=self.compute_bounds(sheet_coord_system).centroid()
  
          b_row,b_col=sheet_coord_system.sheet2matrixidx(bounds_x,bounds_y)
  
          row_offset = cf_row-b_row
          col_offset = cf_col-b_col
          self.translate(row_offset,col_offset)
docs: Offset the bounds_template to this cf's location and store the
          result in the 'bounds' attribute.
  
          Also stores the input_sheet_slice for access by C.