Absolute value operations on vector classes

[AfonsoDGBatista]

Added abs and absLocal methods on Vector2f, Vector3f and Vector4f classes.

[jaime-jmebot]

The six new public methods have no test coverage. jme3-core/src/test/java/com/jme3/math/Vector3fTest.java exercises essentially every other public method of Vector3f (including maxLocal, minLocal, negate), so abs()/absLocal() should get matching tests there. Worth asserting the edge cases the existing suite cares about: negative components, -0.0f (→ +0.0f), and NaN propagation. Vector2f/Vector4f have no test classes, so covering the Vector3f variants is the minimum.

[jaime-jmebot]

• The added logic is correct: abs()/absLocal() follow the engine's ...Local naming, and Math.abs maps -0.0f to +0.0f and propagates NaN consistently with the rest of com.jme3.math.
• Two formatting violations (missing space after a comma) in Vector3f.abs() and Vector4f.abs(); committable one-line fixes are posted inline.
• No tests for the six new methods. Vector3fTest is the established home for per-method vector tests and should be extended to cover the abs variants, including -0.0f and NaN inputs.
• Per CONTRIBUTING.md, non-trivial additions are expected to start with a forum thread; link the discussion in the PR description if one exists.

[JNightRider]

It appears that java.lang.Math is being used. Since FastMath has this same method, shouldn't we take advantage of the FastMath class instead of the Math class?