convert infix expression to postfix expression
use unittest module for unit testing
supported operators include +, -, *, /, %, ()
Usage:
conver(infix, rtype=str)
convert_num(infix, rtype=str)
@parm infix the string of infix expression
@parm rtype the return type
@return postfix expression string as default, otherwise a list continas all operators and operands
NOTE: not support negative numbers yet