refactor GraphFigures
refactor parser
add polymorphic +-*/^ to VarNode
refactor GraphFigures
add temp insert in equation_entry

parser2.py:
optimaze parse method in Parser
add expression method to Parser
add is_priority method to TreeNode and children
fix VarNode __str__
fix VarNode __pow__
fix VarNode __add__
fix VarNode __sub__
Merge branch 'expression_y' into develop