BUG FIX: Fix handling of <objectiveValues> in CPLEX.py

[kylasakerskog]

Fixes #1766

Summary/Motivation:

@ZedongPeng reported a bug in CPLEX.py.
This bug is caused because of XML tag <objectiveValues> in .sol file generated by CPLEX 20.1.0.0.
This also enter the following IF statement in CPLEX.py line 779.

elif tokens[0].startswith("objectiveValue"):

Changes proposed in this PR:

Described new IF statement to handle <objectValues>.

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[blnicho]

This PR has been superseded by #1792