refactor to extract format() function which will be used by printReadableUnit.
extract print method in FormatedUnit class, and apply this method to printReadableUnit.
rename format to formatToReadable, add formatToMinUnit function.
apply formatToMinUnit in printToInch funciton.
refactor, rename printToInch to printWithMinUnit.
refactor, move the same code to base class (Unit) .
reorder the tasking by my implementation sequence, tasking the operator +.
rename Length:printToInch to printWithMinUnit; add new test for length + operator.
modify length:operator+ 's return type.
move operator + to unit class, get out the implementation and pass the test.
pass the new tests of Unit:operator +.
refactor, move the Unit:Operator + to FormatedUnit:Operator +.