Allow adding constraints

[lbluque]

Summary

Allows adding constraints to underlying cvxpy problem, following suggestion in #17

Checklist

• [X] Code is in the standard Python style. Run pycodestyle and flake8 on your local machine.
• [X] Docstrings have been added in the Google docstring format. Run pydocstyle on your code.
• [X] Tests have been added for any new functionality or bug fixes.
• [X] All existing tests pass.