Added setLength() method to Vector2, Vector3 and Vector4

[ArgentiApparatus]

Added a setLength() method to Vector2, Vector3 and Vector4, both 32 and 64 bit versions.

This can be viewed as a generalized version of normalize(), an I've tried to make the implementations very similar. the behavior when the existing length==0.0 is the same.

[ArgentiApparatus]

A setter was what I originally planned but having looked at other methods (esp. normalize() that return [this] I decided that a function that also returned [this] was more inline with the current code style.

However, I've now (perhaps belatedly) read the comments on your current pull request,and seen that you are removing call chaining. If that is accepted I'd prefer to implement this as a setter.

[johnmccutchan]

I would prefer a setter because we will move away from returning 'this'

[Fox32]

This can be closed, the feature was implemented as a setter in #123