test addition

this test addition of two fractions in the expression
testing of "-", "*" and "/"
added abs check
testing abs
add negate, clear, whole number and quit checks/operations

add negate, clear, whole number and quit checks/operations
In whole number check I use isNumeric method which checks if string is
numeric before proceeding with the IF statement and further operations
The code repeats itself so I might try to fix this later
In quit check I use a newly created method quit () which prints out
goodbye and uses System.exit ()..