Better configuration of Maven Checkstyle.

Adds custom configuration of style_checks.xml, tried to fit it to current style.

Additionally, adding <reportSets> element works around bug where checkstyle generates an additional aggregate report with duplication of elements.

Down to ~1600 Checkstyle warnings now. Running the eclipse formatter configured from the Checkstyle config reduces it to ~900, mostly naming convention issues and magic numbers.

Signed-off-by: bolte-17 <bolte.17@gmail.com>