code: def positionlesscrop(self,x,y,sheet_coord_system):
          
          slice_inds = self.findinputslice(
              sheet_coord_system.sheet2matrixidx(x,y),
              self.shape_on_sheet(), sheet_coord_system.shape)
  
          self.set(slice_inds)
docs: Return the correct slice for a weights/mask matrix at this
          ConnectionField's location on the sheet (i.e. for getting the
          correct submatrix of the weights or mask in case the unit is
          near the edge of the sheet).