fix histograms for TIV variable

[Marigold]

• Resolve Python-histogram fails on TIV variable #10
• Add unit tests

"PARAM_query=SELECT \"tiv\",\"dataset\",\"gender\",\"agegroup\",\"alzheimerbroadcategory\" FROM cde_features_a WHERE \"tiv\" IS NOT NULL",

returns no results because all tiv values are NULL. This PR fixes that and returns error to job_result table.

[ludovicc]

Yes, tests!

[Marigold]

With this change, call to /mining/job is returning

{
    "algorithm": "python-histograms",
    "error": "Cannot handle job results of shape ch.chuv.lren.woken.core.model.Shapes$highcharts$@15b89cc4 produced by function python-histograms with data Some(), error Some(Dependent variable tiv is empty.)",
    "jobId": "cf21e908-0f9a-41e6-9472-f71e7dbcfcb9",
    "node": "federation",
    "shape": "text/plain+error",
    "timestamp": 1521319557
}

is that acceptable?

[ludovicc]

No, you are returning an error message and mark it as a highchart json. It should have been a json that defines a highchart graph saying that there is no data for the variable. I will have a look and fix it

[Marigold]

Thanks for correcting me, I've changed the output type to Shapes.ERROR and woken now returns

{
    "algorithm": "python-histograms",
    "error": "Dependent variable tiv is empty.",
    "jobId": "270a4d45-7e69-4a48-8572-82a36cac87e2",
    "node": "federation",
    "shape": "text/plain+error",
    "timestamp": 1521368504
}

I hope it's correct now.

[Marigold]

Closing this in favor of #13 with distributed mode.