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.

wpimath/src/main/java/edu/wpi/first/math/SimpleMatrixUtils.java

@@ -216,7 +216,6 @@ public final class SimpleMatrixUtils {
    * @throws RuntimeException if the matrix could not be decomposed (ie. is not positive
    *     semidefinite).
    */
-  @SuppressWarnings("PMD.AvoidThrowingRawExceptionTypes")
   public static SimpleMatrix lltDecompose(SimpleMatrix src, boolean lowerTriangular) {
     SimpleMatrix temp = src.copy();