API: figure out where object3d should be in constructors

Summary

Giro3D offers a way to customize which object3D will be at the root of the mesh tree of a particular entity.

But there's discrepancies between the superclass Entity3D and subclasses:

• The former declares it as an independant contructor args
• at least Map and FeatureCollection have object3D in their options parameter

Suggestion: have it in the same position for superclass and subclasses, to be less surprising.

Informations

• Giro3D version: 0.28.0