Add dipole moment observable to hf

[soranjh]

Context:
This PR adds functions required for constructing a differentiable dipole moment observable.

Description of the Change:

• The function moment_matrix is added to qml.hf.matrices for constructing a multipole moment matrix of integrals for a set of basis functions.

• The dipole module is created in qml.hf to host the following functions:
  dipole_integrals: computes the dipole moment integrals in the MO basis
  fermionic_dipole: computes the fermionic dipole moment
  dipole_moment: computes the qubit dipole moment
  fermionic_one: creates a fermionic operator from one-particle MO integrals
  qubit_operator: converts a fermionic observable to a PL qubit observable

Benefits:
This functionality is required for computing a differentiable dipole moment observable and to remove using external qchem libraries for computing dipole moment.

Possible Drawbacks:
NA

Related GitHub Issues:
NA

[github-actions]

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

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[codecov]

Codecov Report

Merging #2173 (26cef3d) into qchem_refactor (ae6f984) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           qchem_refactor    #2173   +/-   ##
===============================================
  Coverage           99.26%   99.26%           
===============================================
  Files                 231      232    +1     
  Lines               18253    18350   +97     
===============================================
+ Hits                18119    18216   +97     
  Misses                134      134           

Impacted Files	Coverage Δ
pennylane/hf/__init__.py	100.00% <100.00%> (ø)
pennylane/hf/dipole.py	100.00% <100.00%> (ø)
pennylane/hf/matrices.py	100.00% <100.00%> (ø)

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[ixfoduap]

Overall looks great to me, I didn't find any major issues. Only points to address are:

• let's think if we can find a better name than fermionic_one
• I believe we should at least try to add gradient tests for the operators

[obliviateandsurrender]

Looks good to me. Left a few doubts and comments.

[agran2018]

Thanks @soranjh ! Nice docstrings. I have left few suggestions.

[agran2018]

Thank you @soranjh. Looks good! I have just left few minor comments.