please clarify quaternion conventions in org.apache.commons.math.geometry.Rotation

Key: MATH363
URL: https://issues.apache.org/jira/browse/MATH363
Project: Commons Math
Issue Type: Improvement
Affects Versions: 2.0
Environment: all
Reporter: Albert Huang
Priority: Minor
Please clarify which quaternion convention is used in the Rotation documentation, and provide
constructor methods to use alternate quaternion conventions.
For example, the computer graphics community has standardized on representing a rotation of
angle theta about the unit vector (i, j, k) with the quaternion
{ cos(theta/2), i * sin(theta/2), j * sin(theta/2), k * sin(theta/2) }
e.g., any introductory computer graphics textbook referring to quaternions will describe them
in this convention. This is also the convention appearing on Mathworld, Wikipedia, etc.
The convention in org.apache.commons.math.geometry.Rotation instead appears to be
{ cos(theta/2), i * sin(theta/2), j * sin(theta/2), k * sin(theta/2) }
Since I'm not likely to convince you to switch conventions, please at least document the convention
and provide ways to convert from the alternate (this should be trivial?)

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