TCP server: better error handling

[alphaville]

Addressing issue #61

TODOs:

• [x] Fix malformed JSON
• [x] Improve error reporting (return informative message); currently, only an error code is returned
• [x] Write tests in Python
• [x] Update docs
• [x] Update CHANGELOG

[korken89]

We should also look into providing the starting guess of the decision variables, but that is probably for another PR :)

[alphaville]

@korken89 You can provide the initial guess as follows:

{
    "Run":
        {
            "parameter": [1.0, 2.0],
            "initial_guess": [5.0, 6.0]
        }
}

[korken89]

I mean in the python tcp interface :)

[alphaville]

@korken89 It's supported in the Python interface too. Having constructed an instance of OptimizerTcpManager, use call with two arguments. The first argument is the parameter and the second (optional) argument is the initial guess. For example:

mng = og.tcp.OptimizerTcpManager('python_test_build/tcp_enabled_optimizer')
mng.start()
solution = mng.call([1.0, 50.0, 100.0], [0.0, 1.0])
# or...
solution = mng.call([1.0, 50.0, 100.0], initial_guess=[0.0, 1.0])

[alphaville]

@korken89 I don't know happened, but without changing anything, Travis fails all tests with Python versions <3.6 - see for example this log. There, throughout lines 1012-1030, we see that travis fails to install numpy and gives the error

RuntimeError: Python version >= 3.5 required.

I would be OK with removing all support for Python 2.7 (soon to retire), but neither version 3.4 works.

I tried to do the same locally with Python 3.4 (using virtualenv) and I reproduced the error.

---

Update: I got the following message:

DEPRECATION: Python 3.4 support has been deprecated. 
pip 19.1 will be the last one supporting it. 
Please upgrade your Python as Python 3.4 won't be maintained after **March 2019** (cf PEP 429).

Apparently, Python 3.4 is indeed deprecated, so it makes sense to remove support for it.

[korken89]

Oh, I'd remove it from the CI as you have done now

[alphaville]

@korken89 I added a clarification in the docs that Python >= 3.5 is required. You need to approve this if you agree with these changes so that I can merge it in master.