Improved implementation and unit test for Vector3.angleTo to account for numerical problems outside of full [-1,1] range.

Fixed bug in Vector3.angleTo where NaN was being returned instead of zero. Added corresponding unit test.

Removed Color.getHSV, Color.setHSV and ColorUtils.adjustHSV. Added Color.offsetHSL. Updated some examples...

remove Vector3.projectOnUnit, add Vector3.projectOnPlane (equivalent to previously removed orthoTo).

Plane: Removed normalize() from setFromNormalAndCoplanarPoint and added it back to setFromCoplanarPoints. As suggested in 5c6b5746.

update math constructors to not copy when provided explicit initial values per @mrdoob: https://github.com/mrdoob/three.js/pull/2902#issuecomment-11999928

Frustum bug fix: clone and constructor should be real copies.  Fix bug in unit test to catch this in the future.

Removed Vector* add() to addVectors() and addSelf() to add(). Same with sub, cross, min, max, lerp... Renamed Matrix* and Quaternion multiply() to multiplyMatrices(), multiplyQuaternions() and multiplySelf() to multiply(). See #2860.
Good thing we have a ton of examples to make sure nothing is broken. The unit tests also helped. However, I suspect I've left something unchanged.

Also, code clean up here and there.

Add missing space
Fix isIntersectionSphere
Addunit test for isIntersectionSphere

Frustum.contains -> intersectsObject, containsSphere->intersectsSphere, removed Frustum.containsAnyPoints, .transform.  Update unit tests accordingly.

fix ArrowHelper so that it can deal with normals pointing down [0,-1,0] via creating a unit test (makeBasis).

expand Matrix3 with set, identity, copy, multiplyScalar, deteminant, clone;  Matrix3/4.getInverse will now optionally throw an exception; start Matrix3/4 unit tests

inline Vector2/3/4/Quaternion.length()/lengthSq() and optimize Vector2/3/4.setLength() with unit test coverage.

remove duplicate Quaternion.toEuler (already on Vector3.setEulerFromQuaternion), expand Quat unit test, fix merge.

rename benchmarks and unit test HTML files based on pattern established in /examples.  Expand float23array benchmark to cover most alternatives.