Changes
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* `BinaryQuadraticModel` now subclasses `AdjDictBQM`
* Remove `BinaryQuadraticModel.to_coo` and `BinaryQuadraticModel.from_coo`
* Remove `BinaryQuadraticModel.to_numpy_matrix` and `BinaryQuadraticModel.from_numpy_matrix`
* Remove `BinaryQuadraticModel.SPIN` and `BinaryQuadraticModel.BINARY`
* Remove `vartype` keyword argument from `BinaryQuadraticModel.add_variable`, `BinaryQuadraticModel.add_variables`, `BinaryQuadraticModel.add_interaction` and `BinaryQuadraticModel.add_interactions`
* The `.spin` and `.binary` properties of binary quadratic models are now views rather than copies
New Features
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* `as_bqm` function for constructing binary quadratic models
* Most `BinaryQuadraticModel` methods and properties have been added to `AdjArrayBQM`, `AdjDictBQM`, `AdjMapBQM` and `AdjVectorBQM`