r.describe: Add JSON support

[NishantBansal2003]

fixes: #4903
Use parson to add json output format support to the r.describe module.
The JSON output looks like as follows:

{
            "null_value": "*",
            "ranges": [
                {"min": -5, "max": -4.96078431372549},
                {"min": -4.529411764705882, "max": -4.490196078431373},
                {"min": -4.019607843137255, "max": -3.980392156862745},
                {"min": -3.5098039215686274, "max": -3.4705882352941178},
                {"min": -3.0392156862745097, "max": -3},
                {"min": -2.5294117647058822, "max": -2.4901960784313726},
                {"min": -2.019607843137255, "max": -1.9803921568627452},
                {"min": -1.549019607843137, "max": -1.5098039215686274},
                {"min": -1.0392156862745097, "max": -1},
                {"min": -0.5294117647058822, "max": -0.4901960784313726},
                {"min": 0.4509803921568629, "max": 0.4901960784313726},
                {"min": 0.9607843137254903, "max": 1},
                {"min": 1.4705882352941178, "max": 1.5098039215686274},
                {"min": 1.9411764705882355, "max": 1.9803921568627452},
                {"min": 2.450980392156863, "max": 2.4901960784313726},
                {"min": 2.9607843137254903, "max": 3},
                {"min": 3.431372549019608, "max": 3.4705882352941178},
                {"min": 3.9411764705882355, "max": 3.980392156862745},
                {"min": 4.450980392156863, "max": 4.490196078431373},
                {"min": 4.96078431372549, "max": 5},
            ],
}

[NishantBansal2003]

Hi everyone, could someone please review this PR when you have a moment? Thank you!

[echoix]

About the format shown in the body of your PR: Does this mean that the root of the json output is an array instead of an object (like {})? I didn't know it was valid json. I see that the similat shape is used in the test.

For your PR, I don't think I'm able to judge about the C part. But for the Python and test part I could check out.

[NishantBansal2003]

About the format shown in the body of your PR: Does this mean that the root of the json output is an array instead of an object (like {})? I didn't know it was valid json. I see that the similat shape is used in the test.

For your PR, I don't think I'm able to judge about the C part. But for the Python and test part I could check out.

Yes, the format will be an array of objects, each containing a category value (originally separated by newlines in the plain format). However, if this format is not appropriate, we can discuss and consider a new output format.
Sure, please take a look at the Python and test parts.
Thanks!

[NishantBansal2003]

Hey @petrasovaa, I tried following the suggestion from #4903 (comment) with some minor adjustments. Could you take a look?

[petrasovaa]

Hey @petrasovaa, I tried following the suggestion from #4903 (comment) with some minor adjustments. Could you take a look?

That was fast! This looks great, I will test it more tomorrow and let you know if anything else is needed.

[petrasovaa]

I finally tested it and the only thing I would change is the null reporting, it seems to me that a boolean like "has_nulls" makes more sense here for the JSON, so I would ignore the nv parameter.

The test would be normally better done as a separate PR before r.describe is modified, but I tested the plain format parts of the test without the changes so it's fine as it is now.