equals() should (probably?) be symmetric

[hcientist]

See some discussion in CS 159 planning (sorry to the folks who aren't JMU CS affiliated for posting the private link). It seems that we should likely:

1. teach the use of Object.getClass() for use in implementing equals methods that @Override Object.equals(Object obj
2. teach that usually implementations of equals methods should be symmetric, and so instances of a subclass should not be considered equal to instance of their superclass

Activities affected:

1. Act11 - Designing Classes, e.g. Color.java and Point.java
2. Act 19 - Extending Classes? Not sure about this one. The current implementation of BigInteger.equals(Object x) doesn't seem to check precisely that the paramter is specifically a BigInteger (i.e. it would permit an instance of a subclass, as explored in this activity).
3. Act20 - Polymorphism, e.g. Person.java and Student.java