Altitudes are nullable in positions but could not be read because deserialization was done into double[].

This PR fixes this by reading positions using the JSON reader directly, token by token. As a consequence, benchmark shows that reading positions are also slightly faster.

Unit tests are added, not only for reading altitudes but also for reading string literals and numbers as strings (added in PR #19).

Example of GeoJSON that was failing before, but works with the changes in this PR. Note altitude "null" at coordinate 3.

{
    "coordinates": [
        [4.8892593383789062, 52.370725881211314, 10.0],
        [4.8952674865722656, 52.3711451105601, 10.5],
        [4.8920917510986328, 52.369310952782627, null],
        [4.8892593383789062, 52.370725881211314, 10.2]
    ],
    "type": "LineString"
}