Automate copyright header checks for Java source files

(Credit to @atuzhykov )

There's (at least) 2 options:
(a) we can probably check the source during tests, scan all .java files and check first few lines for copyright header
(b) use a maven plugin (run during tests? or build?) that fails if copyright headers are missing - https://www.mojohaus.org/license-maven-plugin/check-file-header-mojo.html