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:

[
	{
		"value": "*"
	},
	{
		"value": "-5.000000 thru -4.960784"
	},
	{
		"value": "-4.529412 thru -4.490196"
	},
	{
		"value": "-4.019608 thru -3.980392"
	},
	{
		"value": "-3.509804 thru -3.470588"
	},
	{
		"value": "-3.039216 thru -3.000000"
	},
	{
		"value": "-2.529412 thru -2.490196"
	},
	{
		"value": "-2.019608 thru -1.980392"
	},
	{
		"value": "-1.549020 thru -1.509804"
	},
	{
		"value": "-1.039216 thru -1.000000"
	},
	{
		"value": "-0.529412 thru -0.490196"
	},
	{
		"value": "0.450980 thru 0.490196"
	},
	{
		"value": "0.960784 thru 1.000000"
	},
	{
		"value": "1.470588 thru 1.509804"
	},
	{
		"value": "1.941176 thru 1.980392"
	},
	{
		"value": "2.450980 thru 2.490196"
	},
	{
		"value": "2.960784 thru 3.000000"
	},
	{
		"value": "3.431373 thru 3.470588"
	},
	{
		"value": "3.941176 thru 3.980392"
	},
	{
		"value": "4.450980 thru 4.490196"
	},
	{
		"value": "4.960784 thru 5.000000"
	}
]

[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!