Add grouping to molecular hamiltonian

[soranjh]

Context:
The qml.qchem.molecular_hamiltonian function is modified to support observable grouping.

Description of the Change:
The grouping_type and grouping_method arguments are added to molecular_hamiltonian Args.

Benefits:

Possible Drawbacks:

Related GitHub Issues:
#2996

[github-actions]

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

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[codecov]

Codecov Report

Merging #2997 (9d1855e) into master (f3748c1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2997   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files         269      269           
  Lines       22494    22495    +1     
=======================================
+ Hits        22417    22418    +1     
  Misses         77       77           

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

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[Qottmann]

awesome 💯 that was fast! thanks!