use Double.NaN as default value at WayNode coordinates to avoid confusion

[voomdoon]

org.openstreetmap.osmosis.core.domain.v0_6.WayNode.WayNode(long) initializes nodeId only. latitude and longitude is initializes implicitly, resulting in 0.0

Since 0.0 is a valid value, there is no explicit pointing on incomplete initialization.

To make incomplete initialization visible, the default values should be Double.NaN