Renderers refactoring

[MedericFourmy]

Happypose currently has 2 renderers:

• pybullet (coming from original CosyPose implementation), not used
• panda3d (coming from original Megapose implementation), used for both CosyPose and Megapose

To be able to switch between the 2 and ease the integration of future renderers, refactoring should be a first step

TODO list:

• [ ] : Create a SceneRendererModule abstract class, refactor Panda3dSceneRenderer and BulletSceneRenderer, adapt test_renderer_panda3d.py and write test_renderer_bullet.py
• [ ] : Create a BatchRendererModule abstract class, refactor Panda3dBatchRenderer and BulletBatchRenderer, adapt test_batch_renderer_panda3d.py and write test_batch_renderer_bullet.py
• [ ] Create factory for scene and batch renderers
• [ ] Write CosyPose/Megapose(?) inference unitests for both renderers

[MedericFourmy]

EDIT: the APIs are too different to abstract the inputs. Final changes explained in PR #130