Improved unit test for Modifies & Uses
Fix a bug in ExpressionParser

Bug: Unable to parse expression more than once
Remove testBuildExpressionNode in AST unit test
Added unit test for ExpressionParser

Using AST class buildExpressionNode method
Moved ASTType enum to ASTType header file
Rename getStmtNum to getNumOfStmt in PKB
Improved Parser

Added checking of file before parsing
Added checking of unnecessary token at EOF