replace OpenList with ArrayList

[jbellis]

Making the main interface to external callers use standard Collections makes it easier to use the library.

(it looks like OpenList was intended to optimize performance, but the Java jit is good at inlining things like Arraylist::size now, and even if it weren't, none of the iteration is done in a hot inner loop so the difference is not material.)

[ArlindNocaj]

Hi @jbellis ,

thanks for contributing.

May I kindly ask you for the following two things?

• add the code license statement: "By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice."
• add testcase with 2 examples showcasing that your modifications still work. I see that most of them make sense, e.g. with epsilon, but having 1 or 2 testcases would allow me to be sure that there is no obvious issue.
• Semantic versioning in the pull request title would be great but is optional, e.g. fix: add epsilon comparison and cleanup

After that I am happy to merge it in.

Regards