Add method for tapering gate operations

[obliviateandsurrender]

Context:
This PR adds the final missing piece 🧩 for the tapered variational routine.

Description of the Change:
Adds a function for tapering the gate operations, which are then exponentiated using qml.PauliRot.

Benefits:
One can now taper individual gate operations and build tapered variants of templates.

Possible Drawbacks:
N/A

Related GitHub Issues:
N/A

[github-actions]

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[codecov]

Codecov Report

Merging #3002 (35e4c4c) into master (bebc2f4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3002   +/-   ##
=======================================
  Coverage   99.67%   99.67%           
=======================================
  Files         273      273           
  Lines       23138    23175   +37     
=======================================
+ Hits        23062    23099   +37     
  Misses         76       76           

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

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[obliviateandsurrender]

Based on the feedback, I have updated the previous taper_excitation method to a more general taper_operation method which can be used on any arbitrary gate.

[Jaybsoni]

Hey, left some comments, some suggestions and some small fixes.

[Jaybsoni]

Looks good to me 👍🏼, Nice work!