Added base DFT primitive

[aaronrockmenezes]

Description

Fix #(issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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Checklist

• Run yapf -i <modified file> and check no errors
• Run flake8 <modified file> --count and check no errors
• Run mypy -p deepchem_server and check no errors
• Run mypy -p py-ds and check no errors
• I have performed a self-review of my own code
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• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New unit tests pass locally with my changes
• I have checked my code and corrected any misspellings

[rbharath]

@aaronrockmenezes Make sure to add this to rst docs. Coordinate with @vsaravind01 on which rst files. I want a documentation structure similar to DeepChem where the docs provide scientific comment.

We should add a primitives.rst file as a first cut