Merge qchem and hf modules

[soranjh]

Context:
This PR merges the qml.qchem.hf and qml.qchem modules.

Description of the Change:
All the modules inside qml.qchem.hf are now brought inside qml.qchem and the original qchem modules obs.py and structure.py are merged into a new module openfermion_obs.py.

Benefits:
The changes are necessary for creating a unifies qml.qchem subpackage.

Possible Drawbacks:
NA

Related GitHub Issues:
NA

[github-actions]

Hello. You may have forgotten to update the changelog!
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.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #2385 (5502bb3) into master (5674dbc) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2385      +/-   ##
==========================================
- Coverage   99.42%   99.42%   -0.01%     
==========================================
  Files         245      243       -2     
  Lines       18884    18864      -20     
==========================================
- Hits        18776    18756      -20     
  Misses        108      108              

Impacted Files	Coverage Δ
qchem/pennylane_qchem/qchem/basis_data.py	100.00% <ø> (ø)
qchem/pennylane_qchem/qchem/basis_set.py	100.00% <ø> (ø)
qchem/pennylane_qchem/qchem/hartree_fock.py	100.00% <ø> (ø)
qchem/pennylane_qchem/qchem/integrals.py	100.00% <ø> (ø)
qchem/pennylane_qchem/qchem/matrices.py	100.00% <ø> (ø)
qchem/pennylane_qchem/qchem/molecule.py	100.00% <ø> (ø)
qchem/pennylane_qchem/qchem/observable_hf.py	100.00% <ø> (ø)
qchem/pennylane_qchem/qchem/__init__.py	100.00% <100.00%> (ø)
qchem/pennylane_qchem/qchem/dipole.py	100.00% <100.00%> (ø)
qchem/pennylane_qchem/qchem/hamiltonian.py	100.00% <100.00%> (ø)
... and 3 more

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