Replace indexing with slicing in qchem integrals

[soranjh]

Context:
The qchem code for computing molecular integrals needs to be modified for a better compatibility with JAX as described here.

Description of the Change:
The function integrals.py is modified to replace indexing with slicing in the computationally expensive functions electron_repulsion and _hermite_coulomb.

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[soranjh]

[sc-43000]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c899d6d) 99.64% compared to head (3af5fcc) 99.64%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4685      +/-   ##
==========================================
- Coverage   99.64%   99.64%   -0.01%     
==========================================
  Files         380      381       +1     
  Lines       34275    34056     -219     
==========================================
- Hits        34154    33934     -220     
- Misses        121      122       +1     

Files	Coverage Δ
pennylane/qchem/integrals.py	100.00% <100.00%> (ø)
pennylane/templates/state_preparations/__init__.py	100.00% <100.00%> (ø)
...lane/templates/state_preparations/cosine_window.py	100.00% <100.00%> (ø)

... and 42 files with indirect coverage changes

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[maliasadi]

Thanks @soranjh! It looks really good 🎉

[dime10]

Seems like a straightforward change 👍