Added capability for evaluate method to deal wit hleading and trailing blanks
Added more tests to test abs, negate, clear, quit and leading and trailing spaces
Added tests checking operator being held properly between runs
Added operator being cleared when quit.
Add method to check if something is a valid number and used this in evaluate method as is clearer as also will deal with erroneous input.
Made isValidNumber() private.
Added tests for isValidNumber()
Minor formattting changes made to be clearer.