Add examples
new file:   ch03/ch03x07/Conversion.java
new file:   ch03/ch03x07/Promote.java
new file:   ch03/ch03x08/Array.java
new file:   ch03/ch03x08/AutoArray.java
new file:   ch03/ch03x08/Average.java
new file:   ch03/ch03x09/Matrix.java
new file:   ch03/ch03x09/ThreeDMatrix.java
new file:   ch03/ch03x09/TwoDAgain.java
new file:   ch03/ch03x09/TwoDArray.java