`BQM.from_cqm` or similar

[arcondello]

The inverse of CQM.from_bqm added in https://github.com/dwavesystems/dimod/pull/936.

Cases this would need to handle to be fully featured:

• [ ] Integer variables
  • [ ] binary encoding
  • [ ] one-hot encoding
• [ ] Linear equality constraints over binary variables (can be handled by BQM.add_linear_equality_constraint)
• [ ] Linear inequality constraints over binary variables (can be handled by BQM.add_linear_inequality_constraint)
• [ ] Quadratic equality constraints over binary variables
• [ ] Quadratic inequality constraints over binary variables