Make Molecule the central object for qchem functions

[ddhawan11]

Context:

Description of the Change:
Molecule is the central object for molecular_hamiltonian function

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[soranjh]

Left some comments for now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.68%. Comparing base (a7cd619) to head (8ebf71d).
Report is 15 commits behind head on qchem_feature_branch.

Additional details and impacted files

@@                   Coverage Diff                    @@
##           qchem_feature_branch    #5571      +/-   ##
========================================================
- Coverage                 99.69%   99.68%   -0.02%     
========================================================
  Files                       412      412              
  Lines                     38581    38386     -195     
========================================================
- Hits                      38465    38264     -201     
- Misses                      116      122       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[austingmhuang]

Great work @ddhawan11 . LGTM

[obliviateandsurrender]

Remember to add a changelog unless you'll be adding a combined one in the feature branch.