Modify UI for qubit tapering functions

[soranjh]

Context:
This PR renames the qubit tapering functions, removes a wrapper function used for generating symmetries and PauliX ops and makes the general-purpose user-facing functions taper, symmetry_generators, paulix_ops accessible with qml.functiion_name.

Description of the Change:
The following functions are renamed:

get_generators  ->  symmetry_generators
generate_paulis -> paulix_ops
transform_hamiltonian -> taper
transform_hf -> taper_hf

The functions taper, symmetry_generators, paulix_ops are added to pennylane.__init__.

Benefits:
The UI is more intuitive and there is flexibility in bringing some general-purpose functions out of the chemistry modules without changing the UI.

Possible Drawbacks:

Related GitHub Issues:

[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.
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[codecov]

Codecov Report

Merging #2426 (ec762ee) into master (7d1dd86) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           master    #2426    +/-   ##
========================================
  Coverage   99.47%   99.47%            
========================================
  Files         244      245     +1     
  Lines       19327    19489   +162     
========================================
+ Hits        19225    19387   +162     
  Misses        102      102            

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

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[obliviateandsurrender]

Thanks @soranjh! Leaving a couple of small changes after which it should be good to go!

[obliviateandsurrender]

Looks good to me. Just left one final comment.