Make Cornerstone Tools more extensible

[md-prog]

Explained by use-case

I was trying to make a small customization of the EllipticalROI tool. So I started with extending the existing EllipticalROITool class. (NOTE: I was using Typescript)

The problem was, I wasn't able to call renderAnnotation() method of the original tool class from the new class and therefore I had to copy the full code of the renderAnnotation() method to the new class. And this was followed by some hacks to import internal functions/constants that are not publicly exported from tools package.

Possible cause and solution?

I think it is because the renderAnnotation() method is defined in the form of renderAnnotation = () => {}. And to be able to call this function from child classes, it'd better be defined in the form of renderAnnotation() {}

Necessity ?

This will help us to extend the existing tool without fully redefining the methods. Particularly useful when we want to add small changes to the existing tools like additional annotations.