Sparse DF for PBC

[mscho527]

Adds support for sparse DF. Only targets Γ-point.

Should go in after #259 ✅
Should go in after fixing bug in #259 ✅ as #261
Should go in after #263

• S_abs for PBC, using periodic images
• Interface pbc auxmol (eri_sparse_DF.py:609)
• aux_e2 to pbc equiv (eri_sparse_DF.py:463)
• LR Coulomb part
• Pass on real part of first TA slice to int trans workers (eri_sparse_DF.py:626 & 642)
• Add tests (loooooooong 1D chain to test pbc in regular CI, 3D system in expensive CI?)
• Expose sparse-DF in kbe