r.univar: add JSON support

[kritibirda26]

Use parson to add json output format support to the r.univar module.

Expected JSON schema:

At the root is an array, each element of which has the stats for a given zone. The percentile option allows the user to request specific percentiles which are written for each zone object as percentile_%d keys.

[
    {
        "zone_number": <int>,
        "zone_category": <string>,
        "n": <int>,
        "null_cells": <int>,
        "cells": <int>,
        "min": <double>,
        "max": <double>,
        "range": <double>,
        "mean": <double>,
        "mean_of_abs": <double>,
        "stddev": <double>,
        "variance": <double>,
        "coeff_var": <double>,
        "sum": <double>,
        "first_quartile": <double>,
        "median": <double>,
        "third_quartile": <double>,
        "percentile_98": <double>
    }
]

[wenzeslaus]

Can you please compare this schema to what db.univar generates? Maybe they don't need to be the same, but if they are not, we need to have a good reason for that.

[kritibirda26]

@wenzeslaus Can you please point me to the db.univar module, I am unable to find it.

[echoix]

@wenzeslaus Can you please point me to the db.univar module, I am unable to find it.

https://github.com/OSGeo/grass/tree/f13b37931af919be04fa14f392a1f9ea9aa183fd/scripts/db.univar

[cwhite911]

Update json output and tests to match v.univar.

[kritibirda26]

@cwhite911 updated.

[cwhite911]

@kritibirda26 nice work!

[nilason]

Code looks good to me, I haven't tested though.