SketchElementImpl::ReadidPositionDirection became new SketchElement

[mimaric]

SketchElement/Minimizer objects are not used anymore. IndexGPU internally relies on SketchElementImpl::ReadidPositionDirection. Its output consists of the content of SketchElementImpl::ReadidPositionDirection split into three separate arrays.

Look into ways to:

1. Change the interface of Index so that SketchElementImpl::ReadidPositionDirection does not have to be split into three arrays
2. Refactor the code to reflect the current state of not using SketchElement objects

[tijyojwad]

@mimaric is this task relevant anymore?

[mimaric]

Point 1 is definitely still relevant, not really sure about point 2