Cleanup `java_implementation`

I did some cleanup of java_implementation:

• I removed the unnecessary caise2019version folder and moved the java source code directly into the java_implementation folder
• I removed all duplicated files (README.md, datacollection.csv, etc.)
• I added a .gitignore to not push any files created by the Maven compiler (bin and target folders)
• I also added the .idea folder to the .gitignore. I'm open to discussions on that one. I don't see the benefit in sharing it, but if someone wants to have it in the repository I can revert that change.