Added plugin to generate Java class hierachy that matches MITRE attac…

[JarnoNiemela]

This pull request mimics attackToExcel plugin, except that instead of Excel the output is a complete Java class hierarchy representing MITRE Att&ck Enterprise, Mobile and ICS matrices.

The new plugin attackToJava, will read the Mitre STIX files of all three domains, and create a fully fledged Java class hierarchy, with own class for each Tactic, Technique and Subtechnique. Thus is to allow easy work with MITRE Att&ck TTPs when working with code that happens to use Java.