test whether a point is inside a `CollisionShape`

[stephengold]

It would be nice to expose isInside() in the Java API, but first I'd need to implement it in C++ for all collision shapes. Although it's declared as part of btPolyhedralConvexShape,isInside() isn't yet implemented for btConvexHullShape or btBU_Simplex1to4.

For inexact shapes, the Java version should automatically take the shape's margin into account.