* Violence in central Nigeria killed 3,000 since 2010 - HRW
* Police ignore witnesses, convictions rare - report
* Police say report untrue, attackers always taken to court
By Joe Brock
ABUJA, Dec 12 (Reuters) - Nigerian authorities have largely ignored sectarian clashes in the nation's religiously mixed central region that have killed 3,000 people since 2010, Human Rights Watch said on...