Add multipole moment integrals to hf

[soranjh]

Context:
This PR adds functions required for computing multipole moment integral over two contracted Gaussian functions. These integrals are needed to construct multipole moment observables.

Description of the Change:
The functions _hermite_moment, gaussian_moment, and moment_integral are added to hf.ientegrals.

Benefits:
This functionality is required for computing dipole moment without using external qchem libraries, which are currently used by pennylane-qchem.

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 #2166 (e20cac4) into qchem_refactor (4fa5fb5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           qchem_refactor    #2166   +/-   ##
===============================================
  Coverage           99.25%   99.25%           
===============================================
  Files                 233      233           
  Lines               18195    18227   +32     
===============================================
+ Hits                18060    18092   +32     
  Misses                135      135           

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

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[ixfoduap]

Overall looks great! Awesome job getting this ready so quickly

I left basically just minor comments. Let's use this PR to already make better choices for function naming

[ixfoduap]

It's a minor thing, but consider writing a better explanation for the return of the hermite_moment function

[agran2018]

Thanks @soranjh. Looks good on my side. I have just left few comments regarding notation.

[obliviateandsurrender]

Looks good to go.

[Jaybsoni]

Just a few stylistic changes, but other then that It looks good to me 👍🏼