Rename variable and documentation revision.
Revise Random[Box|Sphere]PointGenerator

1. Simplify code in RandomBoxPointGenerator::pointOnShape().

2. Fix a bug in RandomSpherePointGenerator::pointInShape().
Revise ShapesPointGenerator

To follow the same structure of DcdCollision and ContactCalculation
Revise UnitTest
Fix a bug in RandomBoxPointGenerator::pointOnShape() when selecting axis
Add UnitTest for ShapesPointGenerator