r.horizon: add distance into output

[petrasovaa]

Distance to the horizon is computed within the algorithm, this only exposes the values with -l flag:

r.horizon --overwrite elevation=circ step=60 coordinates=637505,221755  -l

azimuth,horizon_height,horizon_distance
0.000000,0.197017,5010.039920
60.000000,0.196689,5017.220346
120.000000,0.196969,5014.957627
180.000000,0.197017,5010.039920
240.000000,0.196689,5017.220346
300.000000,0.196969,5014.957627

or

[
    {
        "x": 637505.000000,
        "y": 221755.000000,
        "azimuth": [
            0.000000,
            60.000000,
            120.000000,
            180.000000,
            240.000000,
            300.000000
        ],
        "horizon_height": [
            0.197017,
            0.196689,
            0.196969,
            0.197017,
            0.196689,
            0.196969
        ],
        "horizon_distance": [
            5010.039920,
            5017.220346,
            5014.957627,
            5010.039920,
            5017.220346,
            5014.957627
        ]
    }
]

[nilason]

The code looks good to me!