Disable frivolous PMD warnings and enable PMD in ntcore (#3419)

Some valid warnings like throwing NullPointerException or using a for
loop instead of System.arraycopy() were fixed.

Abstract classes marked with PMD.AbstractClassWithoutAbstractMethod were
made concrete because they already had protected constructors.

Fixes #1697.

cameraserver/multiCameraServer/src/main/java/edu/wpi/Main.java

@@ -94,7 +94,6 @@ public final class Main {
   }
 
   /** Read configuration file. */
-  @SuppressWarnings("PMD.CyclomaticComplexity")
   public static boolean readConfig() {
     // parse file
     JsonElement top;