Initial draft of a data model

[WardLT]

Add a data model to the code base.

To do:

• [x] Load a MOF from the hMOF dataset and store it in these descriptions
• [x] Update interfaces for training tasks accept these data structures.
• [x] Add initial utility operations (e.g., producing a 3D structure of a ligand w/o the linker atoms)

[coveralls]

Pull Request Test Coverage Report for Build 5879517814

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+28.4%) to 76.923%

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Files with Coverage Reduction	New Missed Lines	%
mofa/scoring/base.py	4	73.33%
mofa/generator.py	5	0%
<!--	Total:	9		-->

Totals
Change from base Build 5751397811:	28.4%
Covered Lines:	70
Relevant Lines:	91

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💛 - Coveralls