BeckeWeights.generate_weights should raise an exception when it tries to use Nan radii

[tovrstra]

The following example will cause the becke partitioning to use Nan radii. The resulting grid gives large integration errors but not Nans:

import numpy as np
from grid.becke import BeckeWeights
from grid.molgrid import MolGrid
from grid.onedgrid import GaussChebyshev
from grid.rtransform import BeckeTF
becke = BeckeWeights(order=3)
oned = GaussChebyshev(150)
rgrid = BeckeTF(1e-4, 1.5).transform_1d_grid(oned)
grid = MolGrid.horton_molgrid(np.array([[0, 0, -1], [0, 0, 1]]), np.array([2, 2]), rgrid, 74, becke)
print(grid.weights.min())
print(grid.weights.max())

Output:

/home/toon/miniconda3/envs/horton3/lib/python3.7/site-packages/grid/becke.py:82: RuntimeWarning: invalid value encountered in greater
  alpha[alpha > cutoff] = cutoff
/home/toon/miniconda3/envs/horton3/lib/python3.7/site-packages/grid/becke.py:83: RuntimeWarning: invalid value encountered in less
  alpha[alpha < -cutoff] = -cutoff
-121784734604746.08
109395391879381.38

The resulting grid is not usable, so it would be safer to raise an exception when trying to construct becke weights with atoms for which the radii ar not defined.

[PaulWAyers]

Psi4's radii can be found at:
https://github.com/psi4/psi4/blob/b4a272f74b73df8cf0edc4096081a250c495d55e/psi4/src/psi4/libfock/cubature.cc#L111

They just use H's radius for He. The use 3.3 a.u. whenever it isn't available.

[tczorro]

add an error message to nan after the rollback if exists