Unquantize Points during presimplify

[Sigill]

Simplifying a topology requires the removal of the transform (if any).
This is done it the presimplify() function.

presimplify() only does that on arcs, not points.
If the topology is quantized again later, points will be quantized twice, yielding invalid values.

With this commit, presimplify() also remove the transform of Point & MultiPoint objects.

The code for this fix is heavily inspired by https://github.com/topojson/topojson-client/blob/71e5bd6428fd8b7a40fa980d2ef7862140851f03/src/quantize.js.

For example, here is a topology corresponding to Washington DC, which has been quantized with q=1e4:

{
  "type": "Topology",
  "objects": {
    "subunits": {
      "type": "GeometryCollection",
      "geometries": [
        {
          "type": "Polygon",
          "arcs": [[0]],
          "properties": {
            "name": "District of Columbia"
          }
        }
      ]
    },
    "places": {
      "type": "GeometryCollection",
      "geometries": [
        {
          "type": "Point",
          "coordinates": [5151, 5105],
          "properties": {
            "name": "Washington, D.C."
          }
        }
      ]
    }
  },
  "arcs": [
    [
      [4601, 0],
      [-211, 886],
      [279, 870],
      [174, 437],
      [91, 471],
      [-173, 361],
      [-302, 586],
      [-482, 374],
      [-644, 438],
      [-497, 557],
      [-392, 148],
      [-759, 76],
      [-614, 248],
      [-1071, 1337],
      [1933, 1678],
      [1765, 1532],
      [2918, -2470],
      [3383, -2864],
      [-2801, -2421],
      [-2597, -2244]
    ]
  ],
  "bbox": [
    -77.11978099719528,
    38.80276753027573,
    -76.90931454193253,
    38.99613558029279
  ],
  "transform": {
    "scale": [
      0.00002104875040131482,
      0.00001933873887559323
    ],
    "translate": [
      -77.11978099719528,
      38.80276753027573
    ]
  }
}

The location for the label is quantized:

"coordinates": [5151, 5105]

Running toposimplify -P 0.5 dc.json was causing the coordinates of the label to be quantized twice:

"coordinates": [248381480, 261971438]

[Sigill]

I believe this might fix #14.