code: def kernel(self, spread=1):
          
          # TODO: use self.kernel_type to choose function
  
          def gaussian(data, pixel):
              return mvn.pdf(data, mean=pixel, cov=spread)
  
          return gaussian
docs: This will return whatever kind of kernel we want to use.
              Must have signature (ndarray size NxM, ndarray size 1xM) -> ndarray size Nx1