Remove excessive basis set normalisation

[soranjh]

Context:
This PR removes repetitive basis set normalisation steps in the DHF solver and modifies how the permutational symmetries are applied to avoid repetitive electron repulsion integral calculations. These modifications increase the efficiency of the non-differentiable workflow.

Description of the Change:

• The basis functions are normalised in the molecule class.
• The basis function normalisation in integrals.py functions is skipped if the contraction coefficients are not differentiable.
• The repulsion_tensor function uses a more efficient method to keep track of permutational symmetries.

Benefits:
The timing for performing Hartree-Fock calculations is improved significantly. The results are for computing the HF energy for water (26 spin-orbitals), ammonia (30 spin-orbitals) and methane (34 spin-orbitals) with the 6-31g basis set.

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

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

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
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[codecov]

Codecov Report

Merging #2850 (87ae8e4) into master (e450d49) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2850   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files         252      252           
  Lines       20993    21004   +11     
=======================================
+ Hits        20917    20928   +11     
  Misses         76       76           

Impacted Files	Coverage Δ
pennylane/qchem/integrals.py	100.00% <100.00%> (ø)
pennylane/qchem/matrices.py	100.00% <100.00%> (ø)
pennylane/qchem/molecule.py	100.00% <100.00%> (ø)

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