Improvments

* Improve readme with suggestions from #47 
* Add slatter constructor
* Add addScaled to vector
* Add mix for double and vectors
* Fix a failing test
* Intersection point of three planes
* Corners of a frustum

I also added a script that automaticly creates the vector_math_64 library out of the default sources
You can use it via: ```dart tool\hop_runner.dart generate_vector_math_64``` ( Maybe we should add this to the readme in a Contributing section, with additional guidelines)

One test is still failing, but I have no idea what it should do.