Add HasCustomChecker field to Problem object.

Starting SRM 617, some problems can have multiple correct answers. This makes our automatic testers in the templates useless for now in those problems, because they would tell you example results are wrong when they might be correct.

First step to have a workaround is to detect those problems. I found the Topcoder API method that returns true if the problem has a custom checker and false otherwise. Added that field to the Problem class.