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.

wpilibc/src/main/native/include/frc/simulation/BatterySim.h

@@ -12,6 +12,9 @@
 
 namespace frc::sim {
 
+/**
+ * A utility class to simulate the robot battery.
+ */
 class BatterySim {
  public:
   /**

wpilibc/src/main/native/include/frc/simulation/RoboRioSim.h

@@ -14,7 +14,7 @@
 namespace frc::sim {
 
 /**
- * Class to control a simulated RoboRIO.
+ * A utility class to control a simulated RoboRIO.
  */
 class RoboRioSim {
  public: