Make use of ERI redundancies

[awf]

The ERI tensor has numerous redundancies:

1. permutation invariances: E_ijkl = E_jikl etc
2. orthogonalities: E_ijkl = 0 for some combinations of (i,j) in some basis sets
3. sparsity: E_ijkl \approx 0 when atoms (ijkl) are far from each other

The electron repulsion integrals are stored in ERI and most of the entries are zero. Furthermore, ERI satisfies an 8x symmetry:

• E_ij.. = E_ji.. # E is a symmetric matrix of ...
• E_..kl = E_..lk # ... symmetric matrices
• E_[ij][kl] = E_[kl][ij] # .. and one more symmetry

This file computes the ERI einsum using a sparse matrix vector multiplication.