Parse (and optionally generate) shorter positions with ambiguity

As reported in https://github.com/ge0rg/aprsdroid/issues/76, the spec allows the removal of whitespace in the longitude according to the ambiguity defined in the latitude, e.g.:

```
4903.__N/07201.75W-
```

This should be supported by the parser in `PositionParser.parseUncompressed`, and optionally be generated by the string formatter of `Position.getDMS`.
