'Returns the second moment of the kernel'

@property
def kernel_var(self):

    if (self._kernel_var is None):
        func = (lambda x: (((x ** 2) * self.norm_const) * self._shape(x)))
        if (self.domain is None):
            self._kernel_var = scipy.integrate.quad(func, (- inf), inf)[0]
        else:
            self._kernel_var = scipy.integrate.quad(func, self.domain[0], self.domain[1])[0]
    return self._kernel_var
