Invalid rescaled energy in greens function

It sometimes happens that e_rescaled exceeds [-1.0, 1.0], triggering a RuntimeWarning and returning np.nan at kpm_funcs.py#L87-88.

For example, it failed with these values for e_rescaled, e, _b and _a:

[-0.97388208 -1.00182976 -0.87335181] [-0.51285752 -0.51406303 -0.50852119] -0.4708495112868156 0.043134592326134265