Support for removing ray shapes from a MultiRayShape instance

[gigcastro]

Desired behavior

The MultiRayShape API only provides AddRay which is sufficient when sensors are being statically defined in a sdf file. But there are situations where it is desirable to dynamically define a set of rayshapes to check collisions towards not-predefined directions.

For example, when simulating a "fake" object/model detection sensor in combination with the LogicalCameraSensor and a set of rays need to be casted towards objects that entered in the frustum. This would speedup computation avoiding casting rays outside the frustum.

There is no way of "clearing" a MultiRayShape and re-define the amount of rays being casted.

Alternatives considered

I had to instance a multirayshape with a pre-defined buffer of N rays and re-use them as batchs in cases where objects in frustum where > N.

A new MultiRayShape can be created each time using PhysicsEngine::CreateShape() / PhysicsEngine::CreateCollision(), but it seems that the engine keeps stacking instances internally and increasing memory usage. It seems that there is no way of detaching a shape/collision from the engine.