Improve & bug fix InstancedGeometry and add new tests.

[Ali-RS]

This should fix the issues with using InstancedGeometry and shadow filter reported on the forum: https://hub.jmonkeyengine.org/t/npe-with-instancednode-and-shadow-filter/44240

• Now if all instances of an InstancedGeometry are removed the InstancedGeometry will be properly detached as well and its internal buffer will be cleared.
• Instead of returning a null bound in InstancedGeometry (when it is empty) it will return a bounding box with zero extents. (following the same approach in Node class)
• Renamed getActualNumInstances to a more meaningful name getNumVisibleInstances
• Fixed a bug in visible instances count calculation that was causing com.jme3.renderer.RendererException: Attempting to upload empty buffer (limit = 0), that's an error when used with shadow filter.
• Two test classes are added that demonstrates using instancing with raycasting and shadow. (thanks @duncanj for providing tests)
• Fixed an issue where the LOD level was being ignored by InstancedGeometry!

[Ali-RS]

Going to merge this in 24h, in case someone still wants to review this.

[stephengold]

I don't understand instancing well enough to comment on the changes. I notice you've added some tests---thank you for that!
A small request: please update the copyright dates of the source files you modify, to at least include 2021.

[Ali-RS]

please update the copyright dates of the source files you modify, to at least include 2021.

Sure, will do.