[NEW] added steps counter with steps
[UPDATE] added arraylist for storing stepslist
[UPDATE] added stepslist test cases
[ADD] added show step test case
Merge commit '0683838cec79ec2ebd228178d6691610d91b4852' into develop

Conflicts:
	src/cal/ExpressionTreeController.java
	src/testCal/ExpressionTreeTest.java

Refactor: extract BuidTree() from ExpressionTreeController to ExpressionTree