Add periodic numerical S_abs

[mscho527]

This PR modifies the grid_S_abs function so that it can take periodic cells. Just like the molecular sparse ERI implementation, the plan is to use this as a guide for the analytical approximate S_abs function.

Some sanity check outputs:

>>> from quemb.molbe.eri_sparse_DF import grid_S_abs
>>> import pyscf
>>> cell = pyscf.pbc.gto.Cell()
>>> cell.atom = "H 0 0 0; H 1 0 0"
>>> cell.basis = "sto-3g"
>>> cell.a = [[2,0,0],[0,20,0],[0,0,20]]
>>> cell.build()
<pyscf.pbc.gto.cell.Cell object at 0x15059183e240>
>>> grid_S_abs(cell)
[array([[1.24198132, 1.02971088],
       [1.02971088, 1.24198142]])]
>>> cell.a = [[20,0,0],[0,20,0],[0,0,20]]
>>> cell.build()
<pyscf.pbc.gto.cell.Cell object at 0x15059183e240>
>>> grid_S_abs(cell)
[array([[0.99999997, 0.49648467],
       [0.49648467, 0.99999997]])]
>>> mol = pyscf.gto.Mole()
>>> mol.atom = "H 0 0 0; H 1 0 0"
>>> mol.basis = "sto-3g"
>>> mol.build()
<pyscf.gto.mole.Mole object at 0x1504cf54a7e0>
>>> grid_S_abs(mol)
array([[0.99999997, 0.49648467],
       [0.49648467, 0.99999997]])

[nwhel]

Looks good!