Raise Error for multiple connections to an output

[danielenriquez59]

Optimizer can only handle 1 output (objective or constraint) per alias, otherwise connection is ambiguous

Pull Request Checklist

• Merge in the latest Develop branch changes to your branch
• Remove .pyc files from your repository:
  • Linux/Mac
    • find . -name *.pyc -delete
  • Windows
    • del /S *.pyc
• Run automatic regression and makes sure everything is passing
• Did you make a new regression test that covers your new code?
• Did you update your docstrings?
• Did you update your headers to include your name and date?
• Do a final compare with the Develop branch to be sure what you're changing

[coveralls]

Coverage decreased (-0.003%) to 90.202% when pulling 42172d0e3e95f90e7fea3df53ca3eea05313b0ca on danielenriquez59:patch-3 into ce6baa683e310a4235290558fb3994988f18a854 on suavecode:develop.