GeoDataPoint: Change GeoDataCoordinates inheritance to containment.

If you run into this, please be reminded of GeoDataPoint's purpose:
"Nevertheless GeoDataPoint shouldn't be used if you just want to store
3d coordinates of a point that doesn't need to be inherited from GeoDataGeometry
In that case use GeoDataCoordinates instead which has nearly the same features
and is much more light weight.
Please consider this especially if you expect to have a high
amount of points e.g. for line strings, linear rings and polygons."

Fixes bounding box calculation.